Born in the Ukraine, Jan Koum and his mother immigrated to California when he was 16 and got an apartment through government assistance. She babysat, he swept floors to make ends meet. When she was diagnosed with cancer, they lived off her disability allowance. He had a knack for computers and got a job working at Yahoo as a security and infrastructure engineer for nine years under Yahoo cofounder and fellow Forbes 400 member David Filo, before leaving in 2007. In 2009 he started WhatsApp, now the world's biggest mobile messaging service, with 600 million users. Facebook bought it for $19 billion in cash and stock in February.