The Web is moving towards a user-centric experience. Whereas a few years ago, it was all about visiting destination sites, now it is about destination sites spitting out data that comes to you, via RSS. The attraction of things like Twitter or Facebook lies in the ambient information that flows out and about, and in your largely asynchronous, largely placeless, largely shallow updates on what your friends are doing. You come to know them deeply not by engaging deeply with them, but by building up pictures of lots of small actions they take.
Compare, for example, the destination-like IRC versus the ambient Twitter. Hardcore Twitter fans use it almost in realtime. They answer people, with their @fred syntax convention. They have a better history, perhaps, because they can search the stream in a way that IRC doesn’t really support. But more importantly, you follow Twitter by filtering it; it’s one big stream, and you take little bits of it out. It is as if IRC were all one channel, and you happened to build an aggregate channel of just the people talking that you wanted to hear.