There's far too many of them, and they're far too annoying. The only "social networking" site I've stuck with is Mibba. I have a Facebook and Myspace account, but I NEVER use them, and I have a YouTube account, but I use that only for saving favorite movies so I can find them easier if I want to watch them again.
Who needs social networking sites anyway? Instant Messaging FTW! ^^
I've got facebook, myspace, myyearbook, twitter, livejournal, and I dunno if deviantart and gaiaonline counts, but I have those too. XD I use them all... eh, whenever I remember, but they're not always too inactive.. >_>
I have a Facebook, which I don't really like. A Twitter for stalking celebrities. And a Tumblr, which I actually really love. It's like a great ranting blog, where you can post things as short as a Tweet, or conversations, or music, or photos, and like, it's my brain exploding onto a page.