I definitely prefer Facebook. MySpace is just too much for me. Facebook is easier to use, and more organized as well. Twitter is just basically updating your status on Facebook, so I don't even bother.
To be honest, I have not heard of any of the other ones before. I just looked up the top 10 social networking sites and those were the ones I got. Of course, Facebook is at the top with 300,000,000 registered users with MySpace coming just below that with 263,920,000 registered members. Surprisingly, Twitter was not on the list. 