According to Nielsen News Wire only about 40% of the system's users come back the following month. More stats at the site with some overdone curve fitting. Twitter has had big growth in the last year, but this kind of stat may mean it will not last.
I continue my Twitter experiment, started early this year. It has led to new connections, interesting content (like this post) and marketing learnings. It seems to have increased readership on this blog, where my tweets often point, by about 30%. But will it last? I still think there is a place for something like, but better crafted than Twitter.