There’s no picture of the moment when everything changed for Kevin Systrom. But if there were, it would look something like this: A lanky, very tall, dark-haired man in his late 20s sits on a bench at the Caltrain commuter station in Palo Alto, California. A sepia tone and weathered patina might underscore the mood of weighty contemplation.

It was early April of last year, and Systrom was waiting for his business partner, Mike Krieger, to arrive from San Francisco. Systrom had just left Mark Zuckerberg’s nearby house and was still digesting the offer that the Facebook founder and C.E.O. had made him: to buy Instagram, the photo-sharing app that Systrom and Krieger had launched just 18 months before. The price Zuckerberg offered was $1 billion—$300 million in cash and the rest in Facebook stock, an especially generous-seeming deal, on the eve of his company’s much-anticipated initial public offering.

The offer was even more impressive given Instagram’s size and age. At the time, it had just 13 employees, operating out of a cramped space in the South Park section of San Francisco. Still, the small crew had managed to attract 30 million iPhone users in just a year and a half by offering a service that allowed a person to quickly upload, prettify through the use of filters, and publish images to the Web for friends to see. A version for Google’s Android mobile operating system had launched the week before, gaining another million users in a single day. What’s more, although the app generated no revenue, it had attracted so much attention from venture capitalists that the start-up had nearly closed an impressive new round of funding at a wildly high valuation of $500 million. Zuckerberg had just doubled that, leaving Systrom with a lot to think about on that train-station bench.

Click. If there ever was a money shot to take for Instagram and Systrom, that was it.

Starting Up

In many ways, life has always been a bit charmed for Systrom, who grew up in the upper-middle-class Boston suburb of Holliston, Massachusetts. Such towns are full of smart, ambitious parents raising smart, ambitious children.

And so it was with Systrom. After high school at the nearby Middlesex boarding school, he made his way to Stanford University, having been drawn toward Silicon Valley early on. “That idea that you could get rich really quickly off of starting a start-up didn’t really exist in Massachusetts, on the East Coast, during that time,” Systrom says.

It certainly did on the West Coast, where he entered Stanford’s class of 2006 and majored in management science and engineering. After completing an internship at the company that would become Twitter, he would work for fewer than three years at Google and then for a little more than a year at a small travel-tip site called Nextstop. Systrom would then strike out on his own with a start-up called Burbn, the first incarnation of what would become Instagram. The game-play location-based service was named, in whimsical Silicon Valley fashion, after his interest in fine whiskeys and bourbons.

The Brazilian-born Krieger had been on an even more traditional techie path, moving to a job as an engineer and user-experience designer at the hot social-media platform Meebo in San Francisco after also graduating from Stanford. Krieger, who had come to the U.S. in 2004 and initially wanted to be a journalist, had majored in “symbolic systems,” an unusual combination of computing and cognitive science, which had also been the major of Yahoo C.E.O. Marissa Mayer and LinkedIn’s co-founder Reid Hoffman.

Krieger, who has become the invisible man of the Instagram story, was, in many ways, its soul, according to Systrom, especially its programming one.