Facebook is awesome for finding people, I just wish there was a way to turn off those applications permanently. So far I found the Swedish exchange student from high school (still as hot as ever...) and my primary school enemy who has turned to Jesus.
Just MySpazz and FuckBake for me - I generally use MySpazz for online persons, although meatspace persons are also welcome there. FuckBake is meatspace only. I also have last.fm, though I don't use it for any sort of socialising, simply as a tool to monitor what I've actually been listening to. My stats are a bit buggered up, however, because it includes tunes that I have to learn for work and hence are played repeatedly (because I forget to shut down the widget) and doesn't include anything not played via the PC, so I'm considering removing it.
I'm on facebook but because back when I joined like 4 years ago it was what all the cool college kids did.
I'm on flickr but I have one friend and need to touch up a shit ton of photos in Lightroom before I upload them but friend me, I'd love to see what people on here come up with. flickr seems like the sort of social network with something of a goal I could get behind at least a little. (here)
I'm on Twitter too but I rarely check it and use it more as a digital note taker i can use anywhere that feeds into my blog. here It lets me send tweets like "Happiness is not a warm gun but a full bookshelf."