I'm fairly new to the world of social networking websites. I joined facebook mid-way through November. A contact wished me happy birthday since he'd seen some photos of Rebecca at a birthday party for someone named Steve and figured I was the Steve. Turns out it was a different Steve, but I just had to go check out those photos -- but wasn't able to without an account. So I caved in and finally signed up.

I've become a bit of a facebook junkie since then, checking requests and notifications first thing in the morning alongside PMs here and regular email.
It's actually come in handy thus far -- I've found out some interesting updates about friends I haven't kept in touch with as much as I'd like to be able to, and I've been able to get back in touch with folks I didn't have up to date contact info about (some high school and college buddies). With so many online gaming folks on facebook it also been pretty fun to see where relationships overlap.
I also joined MySpace at the same time, but I haven't done much there and don't expect to in the future.
I joined another social networking site probably a half year ago (commoncircle.net) at the invitation of a college friend / former co-worker, but that site never got off the ground. It is oriented toward a social justice / environmental protection community. The system doesn't offer much of any value aside from connecting with people, and with the number of partipants being in the 100s (even after at least six months) it is kind of laughable. I still occassionally open and read the emails they send, but don't visit the site. I would consider joining anothe niche social networking site like this if it were done in a way that could generate at least some bare minimum of participation required in order to provide some value (for example if idealist.org did more with individual profiles rather than simply organization profiles).
I might eventually cave in and join LinkedIn as well, although I don't expect that I'd visit it that routinely.
But, over the foreseeable horizen I'm all about facebook.
