Evan Williams on Quora:

I got a request to answer this question, though contrary to common belief, we didn’t actually launch Twitter at SXSW — SXSW just chose to blow it up.

We launched it nine months before — to a whimper. By the time SXSW 2007 rolled around, we were starting to grow finally and it seemed like all of our users (which were probably in the thousands) were going to Austin that year. So, we did two things to take advantage of the emerging critical mass:

1) We created a Twitter visualizer and negotiated with the festival to put flat panel screens in the hallways. This is something they’d never done before, but we didn’t want a booth on the trade show floor, because we knew hallways is where the action was. We paid $11K for this and set up the TVs ourselves. (This was about the only money Twitter’s ever spent on marketing.)

2) We created an event-specific feature, where, you could text ‘join sxsw’ to 40404. Then you would show up on the screens. And, if you weren’t already a Twitter user, you’d automatically be following a half-dozen or so “ambassadors,” who were Twitter users also at SXSW. We advertised this on the screens in the hallways. (I don’t know how many people signed up this way — my recollection is not a lot.)

I don’t know what was the most important factor, but networks are all about critical mass, so doubling down on the momentum seemed like a good idea. And something clicked.