"What are you doing?" By asking this trivial question, Twitter has gotten people talking. "The first thing people usually say is, 'Twitter seems like a completely useless service,'" admits 30-year-old Jack Dorsey, who, with Evan Williams, 36, and Biz Stone, 33, started the San Francisco mobile social network in March 2006 so people could ask their friends that question. While some just don't get it, others can't get enough: The number of users and updates doubles every two weeks.Twitter is unique in that it's a complete message-writing system that connects people by bridging today's main messaging tools. Subscribers can respond from their mobile phones, IM clients and Twitter's website. An open application programming interface also lets users create their own TwitterTools--like "twittervision," which shows where updates are being posted from in real time. Now the only question left to ponder is: Will Twitter eventually twitter out?