Krieger said even though the company now has more than 100 million users, he is still one of just three people maintaining its infrastructure. The company learned early on not to needlessly duplicate the work of others but instead build on it. For a startup, that can mean not building their own storage, computing systems, or user interface components.

"A big part of that is figuring out what you want your company to be good at," Krieger said. In Instagram's case, that was speed. It figured out that users would want to use its app during short breaks such as waiting for a bus, and made it capable of uploading their photos and displaying others' images very quickly, he said.

"You're filling in gaps in these peoples' lives," Krieger said.

The IDG News Service