Twitter is really cool. For a long time, I didn’t understand what it was for, but I’m getting an increased understanding of it. It’s kind of like a room where you can define who’s near you, and nearness is not necessarily transitive. So you can overhear conversations and join into them seamlessly, or hear when someone shouts across the room to you. It’s broadcast IM with a smooth multi-user chat transition.
On top of that, the social expectations of the site are such that talking with people you don’t know is encouraged, which just makes it great at community building, rather than Facebook’s community-recording.