Last spring, Chris Cox, the chief product officer of Facebook, was promoted to also oversee Whatsapp, Messenger, and Instagram. It seemed, at the time, almost like succession planning. If Mark Zuckerberg were to ever leave the company, Cox, his longtime confidante, and a representative of the engineering and product side, would be set up to run it. But today Cox has announced that, after 13 years at the company, he’s leaving.