His obsession with cities—and with programming—never abated. By early 2006, having dropped out of N.Y.U. and bouncing between jobs, he found himself working for a San Francisco software start-up called Odeo, which was going nowhere. One day he proposed an idea to his boss based on a notion that Dorsey had been noodling over for years. He was fascinated by the haiku of taxicab communication—the way drivers and dispatchers succinctly convey locations by radio. Dorsey suggested that his company create a service that would allow anyone to write a line or two about himself, using a cell phone’s keypad, and then send that message to anyone who wanted to receive it. The short text alert, for him, was a way to add a missing human element to the digital picture of a pulsing, populated city.