But the site drew so much traffic that GM began inviting Dr. Dennis to Volt-related events and offering him exclusive interviews with its top executives. Advertisers began paying him for space on the site. And upon completing his daily duties, including treating victims of stroke, Dr. Dennis began making daily posts to the site. Last week, when GM Chief Executive Fritz Henderson announced that the Volt would travel 230 miles per gallon of gasoline in city driving, Dr. Dennis flew in for the press conference. “This Web site has just added a whole new dimension to my life,” he says.