Surfing the Web in his all-white Dumbo loft, Dr. Jay Parkinson, 37, looks like any other young tech visionary. He has a trim beard and thick-framed glasses. He wears slim-fitting black outfits and jaunty scarves. He speaks with a measured, “This American Life”-like cadence. And he’s a firm believer in the utopian promise of the Internet. But Dr. Parkinson’s start-up isn’t a new app or social network. He is a founder of Sherpaa, a Web site that operates like a virtual doctor’s office, examining patients by e-mail and text message.