48. Jan Koum
A billion people each day use Whatsapp, the messaging service Koum cofounded in 2009 with Brian Acton (No. 76). Facebook bought Whatsapp for $22 billion in stock and cash in 2014, turning Koum into a billionaire overnight. Koum and his mother immigrated to California from Ukraine when he was 16. His mom brought pens and Soviet-issued notebooks in her suitcase to avoid paying for school supplies for Koum, who got kicked out of his American high school and had to attend an alternative program. He had taught himself computer networking by the age of 18 and enrolled in San Jose State before dropping out to work at Yahoo, where Acton worked.