In his blog post, Zuckerberg said that Cox had told him several years ago that he planned to move on, but that Cox chose to hold off on leaving until the company made more progress combating misinformation and Russian interference - controversies that erupted in the wake of the 2016 election.
The company does not immediately plan to fill Cox's role, Zuckerberg said, adding that Cathcart, Simo and the heads of Instagram and Messenger will now report directly to him.
"While it is sad to lose such great people, this also creates opportunities for more great leaders who are energized about the path ahead to take on new and bigger roles", Zuckerberg said.
CEO Mark Zuckerberg reiterated that the company is organizing around a "privacy-focused" emphasis on private messaging, which he announced last week. "But after 2016, we both realized we had too much important work to do to improve our products for society, and he stayed to help us work through these issues and help us chart a course for our family of apps going forward", Zuckerberg wrote. He pocketed $310 million in gains from exercising Facebook stock options from 2014 through 2017 alone, according to the company's filings with securities regulators.
Zuckerberg, in a separate post, said Cox had discussed the possibility of moving on "for a few years", but had remained for a transition period.
After CEO Mark Zuckerberg and COO Sheryl Sandberg, Cox is the third highest-ranking executive at Facebook, representing the engineering and product sides of the company.
Facebook isn't hiring another executive to replace Cox.
"This will be a big project and we will need leaders who are excited to see the new direction through", he wrote, signaling that he wasn't "excited" about it himself.
Will Cathcart, vice president of product management, will now lead WhatsApp, and Head of Video, Games and Monetization Fidji Simo will be the new head of the Facebook app, Zuckerberg said.
Founding executives of Facebook-owned Instagram and WhatsApp have also recently left the company.
Zuckerberg said on Thursday that Olivan will now lead the effort to integrate Facebook apps, a key move as the company encrypts conversations on more of its messaging services and makes them compatible.