Jan Koum, founder of WhatsApp, practicing with his trainer Ed Carpio. Photographed at CCK Gym in San Jose on September 4, 2014 for Fortune Magazine 40under40 issuePhotograph by Robyn Twomey for Fortune; Grid Photo: Courtesy of Whatsapp
The Ukrainian-born engineer toiled at Yahoo for nearly a decade with co-founder Brian Acton before traveling South America and eventually developing WhatsApp, the simple, free mobile app that works a lot like traditional text, letting users send and receive calls, video, and pictures, as well as messages. The startup ballooned to hundreds of millions of users—becoming especially popular overseas—and made headlines this February when Facebook—the very employer that turned Koum down for a job years before— bought it for $19 billion. It was the most expensive acquisition ever for a venture-backed startup; Koum famously signed the deal at the former social services office where he once stood in line to collect food stamps.