I have accounts on most of the big ones. Twitter, Facebook. YouTube, Steam and Disqus, if they count.
Not a big fan of them in general. Essentially social networking is a very cheap, easy, and lifeless way to socialize with people. People dig hooking up as "friends" and sharing things, and the sites are certainly useful for that, but generally you're not getting very much in the way of meaningful communication or relationships. Usually the only times that really happens are when the site itself is only a component of your relationship; a means of communicating or networking but not the ONLY means. People who met elsewhere and use the site to stay in-touch, or people who met on the site but say, chat on Skype or play computer games together. Bottom line is... social networking is a very poor substitute for... pretty much every other means of human communication.
You also have very minimal privacy on a social networking site. Everyone's watching.
I use them not because I particularly like them but because a lot of other people use them; I use them primarily for my benefit regarding my website and the other stuff I do online. It's a means of promotion and... well, networking. They're services that people use so I use them to my advantage as many others do. Little more, really. Facebook's been handy keeping in touch with a few friends that have since disappeared from ZD though. ^^