San Francisco startup GitHub has all the hallmarks of the next big social network. The company’s base of 3.6 million users is growing fast, and after raising $100 million last year, GitHub was worth $750 million, at least on paper.

Yet GitHub is not a place for socializing and sharing photos. It’s a site where software developers store, share, and update their personal coding projects, in computer languages like Java and Python.

“It’s a social network, but it’s different from the others because it’s built around creating valuable things,” says GitHub CEO Tom Preston-Werner, whose company has been called “Facebook for geeks.”

GitHub’s mix of practicality and sociability have made it into a hub for software innovation. People log on from around the globe (78 percent of its users from outside the U.S.) to test and tinker with new ideas for mobile apps or Web server software. For Ethan Mollick, an assistant professor at the Wharton School, GitHub is one of a new class of technology platforms, including the crowdfunding site Kickstarter, that allow innovation without the traditional constraints of geography or of established hierarchies. “Virtual communities have more influence on reality now,” he says.