We've all been there, and sometimes, when work gets in the way of expanding our collection of sports cars, the only option is to quit. Jan Koum is the billionaire CEO and co-founder of the messaging service WhatsApp, which was acquired by Facebook four years ago. He's also a member of Facebook's board, and The Washington Post reported on Monday he's leaving the company because he's unhappy with how Facebook handles personal data and encryption. In his farewell message on Facebook, though, Koum merely said WhatsApp has a team that's "stronger than ever" and he is off to grow his collection of "rare air-cooled Porsches" and play ultimate frisbee.