Twitter is a way to post a short message to a wide group of subscribers, and to receive messages posted by a wide group of subscribers.
That’s instant messaging. There’s already a standard protocol for it: Jabber (XMPP).
Why not just use it? Why invent a new protocol?!
Actually, Twitter already does have experimental XMPP access to the full timeline — rather than to individual timelines, or to your friends’ timelines — and you can use it to build things like TweetMaps and TweetClouds and Quotably and…
But Twitter should really be built entirely around XMPP. It shouldn’t be a web app at all, though it could certainly have a web front-end. In case you doubt me, here’s an example Twitter-like service implemented by Process One atop the ejabberd XMPP application server.