Oh, for sure. It'll be interesting, though, to see if there is a "next Twitter". I suspect that Twitter is a creature of the era in which it started, and so that there is no next Twitter in the sense of a global town square. I think it will be more like email's slow slide, with different needs causing fragmentation into different tools and spaces.
That is true, but once it does go down it can go very fast, especially if there is a viable replacement. Digg, Slashdot, MySpace, AOL, Google+ etc. Network effects work to your advantage on the way up, but can work to your disadvantage on the way down.
Tumblr and Flickr are shadows of their former selves.