If Twitter goes away, someone’ll come up with something similar that works in a more distributed fashion, like, some sort of XMPP package that ties into a simple CMS. The Twittering will just be redirected through a Jabber compatible chatting protocol or something.

Then, people will set up Twitter-clone providers and all the popular ones’ll provide the same sort of update stream thing that lets the third parties hook in and do nifty things like reminders and other value added stuff.

That is to say, I don’t think Twitter’s value is in the service but in the idea.