Mr. Systrom wasn’t quite ready to break out on his own.

After graduation, he went to work for Google in neighboring Mountain View. By the standards of his peers, it was considered a good and safe job, though not terribly cool. And he joined years after Google went public, which was too late to make a windfall. He lasted there less than three years and moved on to Nextstop, a travel recommendation site that was founded by former colleagues at Google and was eventually acquired by Facebook. But Mr. Systrom, as Mr. Gurevich recalled, was “antsy.” He had made enough investor contacts from his Stanford days, and by early 2010, he had a germ of a business idea.

His big break, if there was one, came at a party at the Madrone Art Bar in January 2010, with a start-up called Hunch as the host. There he met Steve Anderson, 44, founder of Baseline Ventures and an experienced investor who had by then banked on Twitter. Mr. Systrom pulled out his iPhone and showed him something he was building, called Burbn after his liquor of choice.

As Mr. Anderson recalled it, Mr. Systrom had a prototype and a vague idea. He wanted to build a service that let people share their location with friends, like the popular app Foursquare, with some photo tools attached to it. He was testing the prototype with friends.

“We knew mobile was going to be important, and we knew there was an opportunity to create compelling experiences for mobile devices,” Mr. Anderson remembered of their initial conversations. “But we didn’t know a heck of a lot more than that.”

Mr. Anderson worried about one thing: the echo chamber that can plague a one-person start-up. He suggested that Mr. Systrom find a business partner. Mr. Systrom agreed. Within days, Mr. Anderson wired $250,000 to a newly hatched company, set up by a lawyer whom he had recommended to Mr. Systrom. Mr. Andreessen would soon add $250,000 from his firm. Mr. Systrom could then quit his day job.