This BBC article references a Harvard study on Twitter that shows most users never tweet. This is in spite of the explosive growth Twitter has seen over the last year.
I also like James Robertson's point about the parallel to the blogging hype of prior years. There's definitely similarities between the hype of microblogging and the hype of blogging.
BUT, I think it's worth pointing out that most of the read/write web technologies suffer from the "more users read than write" phenomenon. There have been studies (references anyone?) that have proved this. So, I would say that Twitter and other microblogging tools are no different.