I can read what interesting people have to say without having to 'friend' them
I can discover additional interesting people via RTs, MTs and mentions
Every so often, something interesting will get RTd (or MTd) into my timeline that will lead to me discovering another person to follow. That kind of interconnectedness just isn't there on Facebook, as far as I can see.
I can get involved in conversations with people I don't even follow (yet) via hashtags
Again, sometimes this leads to a new connection, but it might also be that I have a single exchange with someone via a mutual interest in a particular topic. And again, this is about the web-like connectedness of Twitter (to my mind), whereas Facebook is much more linear in my experience.