A friend of my who studies this kind of thing says that Facebook is "sticky" and Twitter is not. People give up on twitter after a few months, but facebook has features that keep you coming back like photos and event invitations.
I know a bunch of kids my age who use twitter infrequently (or started it, and stopped after a few weeks) but use facebook all the time. I'm one of the few I know that actually uses twitter with any regularity, and that's just for following reporters and such. My journalism class had to make twitter accounts one class; I'm pretty sure I'm the only one who actually used it more than the test tweet for the easy A...
