One of the first things I noticed when walking into the lobby on Friday was a big screen showing icons floating from left to right, displaying thought bubbles. This display was courtesy of Twitter, a social networking site that consists entirely of redirecting IMs and text messages to group displays. In practice, this means that people would text or IM their thoughts to Twitter during the sessions and those thoughts would show up on the big screen in the hall outside. They would also be broadcast to friends of those people.
Great, but what on earth is it useful for? In a convention context, it could have been useful if I had been trying to coordinate activities with a small cadre of friends. Suppose that my friends and I are attending different sessions. If my session is not good, I text that fact to Twitter. If it's great, I text that instead. The result: we can loosely coordinate swarming without making disruptive voice calls or individually setting up texting lists. (SXSWi is much looser than academic conferences about changing rooms.)
But there's an analogous but much more direct application. Last night I was at the Lifehacker party and I started getting Twitter traffic reporting on the various parties. The traffic mostly had to do with (a) which parties were lame and (b) which parties had free drinks. Let's call it a "beer swarm." And this use was even better, since texting is much preferable to voice calls in a loud party context and since these texts are applicable to a far wider range of friends. There's your killer app. I don't think that Twitter will take off in an office context, but in contexts where you have to silently coordinate an all-channels network, it fits the bill.