WhatsApp founder Jan Koum grew up on food stamps.

Net worth: $6.8 billion

Koum is now a wealthy man thanks to WhatsApp's billion-dollar sale to Facebook, but the 38-year-old came from humble beginnings. His family, which moved to the U.S. from Kiev, Ukraine when he was 16 years old, struggled with finances and lived on food stamps.

Koum learned how to save money from an early age, and he continues to hold onto his wealth even now. When Facebook bought WhatsApp for $19 billion in February, he wanted Facebook to close the deal before he missed his flight to Barcelona; he bought tickets for that flight using frequent flier miles.