Facebook. When I'm online, I'm always communicating with friends on there. It's very useful for figuring out who's throwing a party when and where, too. Lol
I use Twitter, but it doesn't serve much of a purpose for me since I don't have a cellphone that gets the web (my phone is ancient, 2003). Tweeting on the computer is... blergh. Not so appealing. The only thing I really use my Twitter account for is ranting about random things like how much chiffon frays worse than satin or how much pattern drafting gets on my nerves.
Myspace... I hate Myspace so much. It's a huge jumble, too cluttered to properly navigate, and no one uses it anymore. Not to mention it always got nothing but bad publicity when it was in it's prime. I'll be happy when that site shuts down for good.