That's not quite true. The distinctive element of Twitter is that it's "push" based. Which just makes it lame middleware, or maybe lame multicasting. I'm somewhat surprised that no one (that I've heard of, anyway) is using Twitter as an implementation of wide-area RPC for something. That is, computer-to-computer communication rather than person-to-person.

I guess it's somewhat elitist, but it bothers me that they reinvented the wheel, except they left out the axle, and it's a Reuleaux triangle instead of a circle.