I didn't become interested in social networks until MySpace was obsolete. So I can't really speak on behalf of that.
As for Twitter and Facebook, they each have their strengths. What I like about Twitter is that I can connect with a lot of professionals in a very simple way. I find a great deal of the articles I read from Twitter. Facebook, on the other hand, is a much better way to talk with people I know, be it online friends or friends from school, or family, etc.
Twitter and Facebook aren't competing, if you ask me. Both serve different purposes, and are good for different things.