If you don't personally provide this information on LiveJournal, this is not to say that you're wrong in any way for not doing so. There are literally as many ways to use LiveJournal as there are users of it. If it is technically possible, not against the Terms of Service, and not against the local rules of the places you are operating in (such as communities or the journals of others), you are not Doing It Wrong. (Communities and other users have ways of enforcing behavior in their areas, up to and including removal from the friendslist and banning.) (For those new to this party, I've a previous essay on a few of LJ's Social Rules for interaction.)
This is not a how-to guide or an attempt to prescribe the behavior of others on LiveJournal. Rather, please consider this a resource on some of the likely effects of exercising assorted areas of internet privacy. Each person's privacy needs are different. Each social group's reaction to different exercises of privacy may be different as well, so this essay cannot be considered definitive. While I attempt to observe large parts of LiveJournal as raw information to draw my conclusions from, things may be done differently in parts of LiveJournal that I've never stumbled upon.
Unless you are leaving anonymous comments, or reading only without interacting, you will have a username of some sort -- either a LiveJournal username or an OpenID username from another service. You may choose to specify a name to display in addition to the username. Whether it's in the displayed name, or if it's the username itself, other LJ users typically expect to have a name that they may address you by, whether or not it's the name you were born with or your legal name. If you have a username of i_do_not_like_beets_1338 and your displayed name is "or any other root vegetable", someone is probably going to wind up calling you by your username (underscores and all), your displayed name, "beets", "veggie", or some other variation on the names you have specified. If you have a problem with being addressed by your username, make sure to specify the name that people should address you by. (Stating this in a single entry in your journal may or may not work. I recommend the displayed name, if it's too late to change your username.)
Keep in mind that if you have chosen to use your given name with a certain amount of other identifiable information, people you know face-to-face may be able to identify you if they stumble across your journal, particularly if your given name is rare. Even if you do not plan to use your legal/given/face-to-face name in connection with your journal, be mindful of what details you share. Please do not count on no one you know finding your journal to protect you from people making the connection between you and your journal. That hilarious anecdote about your supervisor confiscating a beachball at work and clobbering a wall clock with it? It's already made it all the way across the workplace, so anyone stumbling into your journal from work is going to know that you work with them. From there it's just a matter of putting together the pieces to figure out which co-worker you are. All it takes is one person stumbling over it completely by accident, and linking ten of their friends to it. This tactic is known as "Security by Obscurity" in the Information Technology industry, and is a really fast way to get IT professionals to start cursing using their really interesting vocabulary. If you're planning on keeping your online and offline identities separate, don't do this.
You are required to share a date of birth with LiveJournal, for the purposes of making sure that you have claimed to be 13 or older (or have a parent's permission to use the site). Additionally, this controls whether you will be able to choose to see content that the owner or the Abuse team has marked as adult. (18+ can choose to view whatever they want. Under-18 cannot.) You are not required to share this with the rest of the userbase; this information is so that LiveJournal will have a private record that you have claimed to be 13 or older, and possibly 18 or older, to cover its own bases.
In the past, maintainers of adult-oriented communities were expected to check the birthdates of their community's members and ensure that all people joining were old enough. Some maintainers will still reject membership from people who have not chosen to make their birth year visible. Currently, communities that are set as adult will not allow someone underage to join them, so presence of a journal in an adult-oriented community does reveal (or at least strongly imply) that you have stated you are older than 18, although exactly how much older is between you and your profile birthday privacy settings. (Disclaimer: since the adult content settings were developed fairly recently in LJ terms, and it is possible to create a community and have people join it and only then turn on the adult content setting, it is possible that some people who have not declared an age may be members of adult-oriented communities. However, this would be entirely without LiveJournal's blessing, and through loopholes.)
People expect to be able to understand the connection between the multiple identities of a person they have met in multiple locations. This can be in multiple independent blogs, on multiple social networking platforms, and offline. If you have made public the same e-mail address in all of the locations, connecting the services is really a no-brainer. If you have made every effort to separate actions in one forum from actions in another forum, it can be alarming if someone else makes public that both are you. LiveJournal's Abuse policies state that sharing the full legal name, address, or phone number of another user can be considered an invasion of privacy on LiveJournal. (Note: I am not part of the Abuse team; if you have questions about the policy, you'd need to contact them.) If you have publicly connected your email address to your legal name, and then used that same email address on LJ, I would not be horribly shocked if someone came to the conclusion that they are you, but it's still a dick move to make the connection for those who don't have the brainz.
People appreciate it if you identify yourself to them if you recognize them from somewhere else. It may be as local as that conversation you had in news the other week. It may be as far-flung as their suspicion that you may be their long-lost friend who they knew through their boyfriend from camp. (Shout-out to Kim!) Sometimes you may realize that the last time you met them, elsewhere, you really did not like them (perhaps you thought they were an idiot in that news conversation, or vice versa). Perhaps you do not want them to realize that they are your boss. There is a very human need for persistence of identity, to recognize others out of the context that you are used to seeing them in, and know that it is really and truly them. There is a reason there are so many jokes about mistaken identity between twins. Humans tend to fear not correctly recognizing someone they should be able to identify.
If someone has multiple identities for purposes of keeping various aspects of their life apart, many people will understand this. If someone has multiple identities for the purposes of being a deliberate dick with one and enjoying an untarnished reputation in the same circles with the other, it is widely considered a public service to out the connection between the two.
LiveJournal has very few options when it comes to sex. One can be male, female, or undeclared. Declaring this to LJ is not the same as declaring it to the world, but the world generally wants to know regardless. As a lingering after-effect of a society with strong gender roles, people will make assumptions about other people based on this, sometimes even when they are actively trying not to. This means that even if you never make a public declaration of whether you are male, female, or something completely different, people will often guess that you are one or the other, make further assumptions about you based on that, and even have the nerve to be cranky if their assumptions are wrong.
As a person who has more than one transgendered friend, and, indeed, more than one transgendered friend that I met prior to them declaring their transition, it is my experience that re-mapping someone in your head like that does take a while, and is a bit of work. Even if you haven't changed, their perception of you may well have, and it may take them some time to get over the shock. They aren't to be excused if they act like jackasses during this time, but it's normal to be surprised if finding out.
LJ gives you the option to upload and use user picture icons (variously called userpics, user icons, occasionally avatars, or, confusingly, upi or THANK FRAK THEY FIXED THE CDN PROBLEM FINALLY). LiveJournal users wind up using these userpics in more ways than just avatars (expressions of themselves as they would like others to visualize them), and use them also to display pretty pictures, to send coded messages, to send not-so-coded messages, to convey a particular mood, to show affiliation, and much much more. However, if your icon looks generally human, then many people will find that they have unconsciously started to use that userpic as an avatar of you. My current default picture is that of a bunch of grapes. If I asked people to form a mental image of me, how many of you would think of a bunch of grapes? How many of you would think of a blue-haired cartoon white girl? How many of you would think of Francine from Strangers in Paradise? How many of you still see synecdochic as a balding man hunched over a laptop killing stupid users with vigor?
While it is not necessary to upload an icon, and it is not necessary that the icon be a photo of you, or even intended to represent you, it helps your readers have an image in their head when they think of you, and helps people be able to visually recognize you when you show up in comments. Just be aware that people will start thinking of the icon as you, visually, and may draw assumptions that you may not have intended if the icon is of a human. Do not assume that because an icon is of someone you and your immediate circle of friends know full well is a fictional character or actor, that everyone is going to know that it isn't you, or who it actually is. While you cannot prevent people from coming to the wrong conclusion just from looking at your icon, you can reduce the amount of misunderstandings by noting of whom the icon is in the icon's comments or keywords.
Unless your public/visible posts are strictly limited, either to the vaguest of pleasantries, or on the sort of strict topic that allows for no personal digressions, your readers will start to hear little snippets about you and your life, and may begin to extrapolate from these small windows on your privacy. Perhaps they are correct; perhaps they are not. Perhaps they will care to share some of their assumptions. Perhaps you will choose to correct them. Perhaps they will prefer the real you to the person they have made up in their heads. For any of these possibilities, perhaps not.
Please remember that exercising a strict control over what details of your personal life you allow out onto the internet is distinct from exploring alternate aspects of your personality in the controlled and remote environment of the internet. This, itself, is distinct from role-playing a person that you very much are not. (It is worthy of note that if you happen to be role-playing a jackass on the internet, unless everyone you interact with is explicitly aware that you are role-playing, perhaps by signs such as a prominent notice on your profile, or by confining this to an appropriate forum such as efw, then you are no longer role-playing a jackass. You are being a jackass. The internet never was solely for your amusement, and people conduct business and parts of their real lives on it in addition to merely playing.)
However you conduct yourself on the internet, remember that anyone who sees you may form conclusions about you based on what of your behavior they do see, and they are under no obligation to get a full or balanced view of you before forming an impression of you. Any action you take may possibly form someone's first impression. Any actions you take on the internet, no matter under what identity or lack thereof, are still actions that you have taken. It does not make you no longer yourself if you put on a mask and a Spandex suit and go out on the street and fight crime, as countless superheroes have already discovered.
No matter how careful you are about sharing your real personal information on the internet, all this goes out the window when meeting someone in person. If you have represented yourself as someone other than who you actually are, face to face, you are going to get called on this if you meet someone face to face. Some people take this better than others. Consider also that the person you are meeting may not have been particularly forthcoming about their actual self. There are countless articles out there devoted to protecting yourself when meeting someone you've never met before from the internet. Even if you don't intend to take all the suggestions if meeting people off the internet, at least acquaint yourself with what the general advice is.
Currently, all icons, friends, communities that you are a member of, and interests are public. Public posts from people you list as friends are public. (Public posts from people who list you as friends are public too, but they are not conveniently aggregated in one place, and people don't always hold one's followers against one, if the relationship is not mutual.) Aside from the sections of the profile that indicate that they may be locked, the entire profile and the information on it is public. If you have shared any information that should not be public with your friends, be sure that they know to not share this; any form of security is only as good as its weakest link, and it is appropriate to ensure that a gossipy friend does not become your weakest link if it is important to you to not share certain information.
One is asked for one's location when signing up for LiveJournal, but this is not required. It can also be kept private. One is prompted for one's location when updating one's journal, and while this location is then linked to a search of Google Maps, this is also strictly voluntary. However, even if you have not declared your location, but you have posted entries, someone who can see these entries may be able to determine your time zone. If you post an entry with the accurate time where you are located, someone may observe this time and compare it to when they noticed it showing up on LiveJournal. "Hmm," they say. "You just posted this, and the timestamp says it's 9 pm. It's 9 pm where I am too! You must be in my time zone."
Other users may log one's IP address when one comments to their entries or in their communities. In those cases, one is shown a warning below the comment field. It is not possible to retroactively obtain IP addresses if the journal owner does not have it turned on at the time the comment is made. Journal owners may obtain IP addresses of visitors in other ways, typically by displaying an image from a server that they control. An IP address by itself shows what internet service provider or proxy one is using to connect to the internet, and, if one's ISP has made this information available, the greater geographic region that this IP address is assigned to. Other people on the internet have collected more detailed statistical information about IP addresses, to attempt to narrow down closer where an IP address has been assigned (this is sometimes dead on, and sometimes way off), or what IP address is likely to go with what LiveJournal username.
Other people have ranted elsewhere about how using IP addresses to verify identity is in no way accurate. The problem with information like this is that it is spotty. Sometimes it is dead on. Sometimes it is totally fucking off. Relying on it to be either is going to cause problems.
To actually identify someone by IP address for certain, you need their IP address, the time (and time zone) when they were in use of that IP address, and probably some warrants. Next you track down the owner of the IP address that they were using. If you're lucky, this was their Internet Service Provider, and they have paid out the nose for a static IP address, and they are the only person who lives there, were home then, and home alone, verified by the five little old nosy ladies living across the street. If you are not lucky, it is an anonymous proxy (good luck and you'll probably need a warrant from whatever country that proxy is in), or a coffee shop that offers free wifi and does not maintain logs of who might have been sitting at one of their tables outside, or (once they tracked down a dynamic IP address to who had it at 6pm on the 23rd of the month) temporarily assigned to a little old lady who has a wireless network but has never secured it and there's a whole apartment building full of teenagers leeching her wireless.
However, if you'd like to claim that it totally wasn't you who made that nasty little anonymous comment from that IP address at 6pm on the 23rd of the month, watch out that you didn't make a comment under your own name on a protected entry at 5:59pm on the 23rd, followed up by another comment under your own name (all from the same IP, mind you) at 6:03pm of that same day. (No lie, I caught out an ex-boyfriend trying that one once.)
While polite behavior should be the norm rather than the exception in many cases, the sad fact of things is that while you are free to insist on polite behavior in your domain, your bare word means nothing without some clout behind it. Your journal is your domain, and you have the right to control the conversation in there. No one has the right to comment to it unless your settings allow them to. You can ban known troublemakers before they have a chance to act up. You can ban new troublemakers once they have established themselves. You can choose to pre-emptively screen all comments, or comments from parties outside of your friends list. You can choose to post to friends-only. You can choose to screen comments after they have been made, or freeze comment threads. You can communicate with your commenters in your journal, or possibly through private message, or any other method they have listed publicly. If someone is a repeat offender and evades a ban, you can take that one to Abuse.
There is a social expectation that if it is not okay for any arbitrary member of the public who happens to come across your entry to comment to it (displaying an appropriate level of civility), then the entry either should not be public (so the general public cannot see it to comment to it) or comments from the public should be screened or disallowed. This expectation exists because the tools to do these things exist.
If an entry is public and the public is not allowed direct comment on it, people will still read it, react to it, and share their reactions and likely a link to the original in their own space. There is a difference between an incitement to harass and a link for context.
It is socially problematic to take an entry that was public and hide it after the public has discovered it. The internet as a gestalt believes that information should be free. If information was on the internet this means the cat has been let out of the bag, and there are people who will take it upon themselves to make a copy of any interesting or entertaining content they come across, either for personal archives or for sharing with others. (If you share things on the internet so that people can see it, people will technologically be able to make a copy of it, even if it has to be by pointing a video camera at their screen. This is no longer the Century of the Fruitbat. Yes, even if you have some stupid script on your website making this a No-Right-Click Zone. LiveJournal, thank fuck, does not allow scripting such as that on people's journals, making support that much easier.) If you have a doubt about whether something is suitable for the public, perhaps consider posting it locked at first, and getting a few second opinions before making it public. (If someone is reproducing your words without your permission, United States law has the Digital Millennium Copyright Act. More information about this as applies to LiveJournal can be found here and here.)
The nature of anonymous commenting is that one jerkwad can spoil it for the rest of the world. If you have a problem with abusive anonymous commenters, at a minimum, you should be logging the IP addresses and screening anonymous comments before they are publicly visible. If an anonymous commenter is still being a jerkwad, please close the barn door. Certain other blog platforms have the ability to block commenting (anonymous or not) by IP address; LiveJournal has chosen to not allow that much refinement in comment settings. (No small part of this is likely due to public misconceptions about the nature of the IP address, and how much of a pain those misconceptions would be to provide support for.) Unlike in years past, LiveJournal accounts no longer require an invite code, and are free to sign up for.
Any online service develops its own personality. LiveJournal has cultural expectations that vary from subculture to subculture. If you barge into a discussion in a LiveJournal subculture that you're not part of, you will probably miss many social cues, and come off looking ... a little odd. (This can range from mere "I look like a n00b" to "I just made a flaming jackass of myself in ways that I have yet to discover".) If you do not own the journal or maintain/moderate the community, this is not your territory. You are a guest, and your presence and voice there is on the sufferance of the actual owner. This is regardless of who happens to be "right" in any dispute.