Mr. Williams, along with the other founders, Biz Stone and Jack Dorsey, first envisioned Twitter as an easy way to stay in touch with people you already know.

In 2006, when Twitter was just starting, the three men felt a small earthquake in San Francisco. They each reached for their phones to twitter about it and discovered tweets from others in the city. At that moment, it dawned on them that Twitter might be most useful for something else — a frontline news report, not just for friends, but for anyone reading.

Indeed, the news-gathering promise of Twitter was most evident during the terrorist attacks in Mumbai last November and when a jetliner landed in the Hudson River in January. People were twittering from the scenes before reporters arrived.

“Twitter reverses the notion of the group,” said Paul Saffo, the Silicon Valley futurist. “Instead of creating the group you want, you send it and the group self-assembles.”

Read that last paragraph again. You can pretty much explain Twitter to anyone from now on using just these two sentences! They contain one of the 3 secrets to Twitter’s success: The open (asymmetric) follower model. (The other two being speed and simplicty…)