Friendship online works differently from friendship in real life. The extent to which it differs may vary based on whether you're an introvert or an extrovert. Here are some quick examples of what constitutes a friend (or a buddy, etc.) in various situations.
Instant messaging: Friends, or buddies, are people who talk to frequently, or at least want to see the online status of. In some networks (such as AOL) you can befriend someone without them even knowing. Basically, your friends are the people in your address book.
Facebook: Friends are anyone you've met before. If you're an attention whore or just want to get the maximal number of friends, your "friends" may even be friends of friends you've never met. You can be looking at anyone on the site, click the button to add them as a friend, and there's basically no chance that the other person won't confirm that you're their friend, unless you're their parent or professor or someone with nefarious sub-goals. Once you're friends with someone, you get a live feed of what's new in their life, which is pretty cool. (Funny how it was controversial and made it to the front page of Slashdot a year ago, and now it's just the way things are.) Coworkers you talk to every day are Facebook friends, long-time friends who have moved to other continents that you never talk to are Facebook friends, and even people you don't really like that much are Facebook friends. "Friend" just connotates a connection of some sort.
World of Warcraft: In World of Warcraft, your friends are either people you know and are actually friends with, or people you've played with in the past who didn't suck. This is pretty different from my previous examples; reputation is involved. You don't add random people you come across to your friends list, and once you've been playing for a while, the list becomes pretty exclusive. You're limited to a pretty low number of friends (maybe 80 or less), so you can't just add everyone to this list—it gets filtered to a list of competent players who are pleasant people to play with. In addition, there's an ignore list, which is basically your anti-friends list. People on this list can't even talk to you anymore, so it's where annoying spammers and bad people and incompetent players go.
Xbox Live: I'm not terribly familiar with the Live reputation system since I don't have any game consoles, but in addition to maintaining a friends list like World of Warcraft, you can give feedback on other players to adjust their global reputation score, so in theory you can know a little something about other players before you even meet them.
The Xbox Live and World of Warcraft definitions of "friend" are very similar, but they're quite different from the other two I mentioned, and those are quite different from what I'd consider a "friend" in real life, and even that is different from what an extrovert might consider a "friend." As a bona-fide introvert, "friend" means something. Friend means that I think that they're a good person... maybe at least 6 or 7 on a 10-point scale, and that I know them at least somewhat well, after spending a reasonable amount of time around the person. It means that, at least to some small degree, I'm willing to put my own reputation on the line to vouch for the quality of that person.
People who meet only some of these qualifications might just be an "acquaintance." I may call them a friend from time to time for simplicity and to avoid hurting feelings, but the term really means something to me.
Online, being someone's "friend" doesn't mean too much. But in real life, for me, it can mean quite a lot. What I wonder is how this is different for other people. In the end, it matters little. It's just a word, and what matters is the relationship that word represents. (Somehow I see V from V for Vendetta saying that line.) What's intriguing is how other peoples' qualifications for friendship differ. I imagine that there are people who don't, for example, feel that another person doesn't have to be what they'd consider a good person to be a friend. Who are your friends? Are there extroverts who regard real friendship as something so relatively insignificant as clicking a link on Facebook? I think there are.
(Not that I have too much against extroverts...)
Currently listening: Jewel—Standing Still, Massive Attack—Antistar