I joined Facebook earlier this week. It's quite fun but possibly serves no useful purpose at all apart from letting you pretend you're friends with loads of people and haven't completely lost touch with them at all, honestly.
I joined Friendster years ago which as far as I can tell is a very similar sort of thing. I asked lots of my friends to join too and most of them refused so it was a bit rubbish - I was clearly well ahead of the times. Now social networking is the zeitgeist and everyone's doing it.
Facebook lets you join Networks of people who
a) were at the same uni as you or
b) work at the same company as you
c) live in the same place as you
I haven't done this yet because
a) you need a valid uni email address - I can't remember mine (it was 10 years ago!)
b) might do this but it feels a bit tragic (it's not like I can't look up people at work already, when I'm at work!)
c) there isn't a network for York
so I remain unaffiliated for now.