Jan Koum cofounded WhatsApp, now the world's biggest mobile messaging service, in 2009. Facebook bought it for about $22 billion in cash and stock in 2014; Koum sits on Facebook's board. Born in the Ukraine, 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. 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 billionaire David Filo, before leaving in 2007. In October 2014 Koum donated roughly $555 million of Facebook shares to the Silicon Valley Community Foundation, which provides grants for local needs, and has diverted some of the funds to several undisclosed causes. Between February and May 2016, Koum donated just over 6 million shares of Facebook to an undisclosed recipient.

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