The nation’s newest billionaire is a rags-to-riches success story who escaped communist Ukraine as a teen and lived on food stamps in the United States before hitting it big with a messaging app that has become the hottest thing in social media. Meet Jan Koum, the 37-year-old college dropout who, with his partner and mentor, Brian Acton, just sold their startup, WhatsApp, to Facebook for $19 billion in cash and stock. Koum is now worth $6.8 billion. It’s a long way from the government-assisted, two-bedroom apartment in Mountain View, Calif., where he came of age with his mom, and even...