A teen wants to be seen and noticed by all the right people. Sometimes that's friends, sometimes it's crushes. They need to control that audience, so that the wrong people don't see things, and so that they know how best to talk to people. Think of it like the cliques in school.
Facebook gives me people I know. MySpace gives me people I know. When I talk, I'm talking to people that I have a relationship with. That means I can talk differently, talk more directly and more emphatically. On Twitter anybody can listen, and that's a bad thing.
(It's still a bad comparison, though. Facebook creates a network with photos and videos and conversations and statuses and messages. Twitter only has statuses. The biggest draw of Facebook is its extensive, localized feature set.)