I am going to go on a ledge here, but some of my behind on the line and make a prediction based on my evolving concepts of the Internet. I think Twitter has peaked, that is its recent growth will suddenly come to a stop.
The key question is if Twitter is Hotmail (stable and essential), Flickr (stagnate and outdated but surviving) or Bebo?
Web 2.0 fads exhibit a radical come and go, they top the "e" generation of the late 1990s in fad and buzz words. So far none has stuck. If you ask me I don't think it will be Twitter. I think the next thing to watch is Last.FM which will take off soon.