Cox’s departure is especially enormous for Facebook, where he was one of the first 15 engineering hires. As the company grew, Cox climbed his way through the ranks, touching every key Facebook product, including its News Feed, and most recently the company’s strategy. Cox gave his notice on Monday, according to a regulatory filing from Facebook.

In a note posted to his Facebook page, Cox said the shift in Facebook strategy toward create a private messaging network across its apps made it the right time for him to step aside.

“As Mark has outlined, we are turning a new page in our product direction, focused on an encrypted, interoperable, messaging network. It’s a product vision attuned to the subject matter of today: a modern communications platform that balances expression, safety, security, and privacy,” he wrote. “This will be a big project and we will need leaders who are excited to see the new direction through.”

Zuckerberg said he appreciates Cox’s “deep empathy for the people using our services and the uplifting spirit he brings to everything he does,” according to a blog post by the CEO announcing the changes. Zuckerberg also announced that WhatsApp Vice President Chris Daniels is leaving the company. Prior to leading WhatsApp, Daniels ran the business development team at Facebook and led Internet.org, Zuckerberg’s philanthropic venture to bring Internet connectivity to the developing world.

“Chris is one of the clearest and most principled business thinkers I’ve met and the diversity of challenges he has helped us navigate is impressive. I’ve really enjoyed working with Chris and I’m sure he will do great work at whatever he chooses to take on next,” Zuckerberg wrote. “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.”

Will Cathcart, who previously was head of the Facebook app, was named the new head of WhatsApp. Fidji Simo, vice president of video, games, and monetization at Facebook, will take over as the head of the Facebook app, Zuckerberg said. As for Cox’s role, Zuckerberg said he doesn’t plan to appoint anyone in the near term, and will instead rely on the heads of each business unit at Facebook.

Cox and Daniels are the latest executives to leave Facebook, as the company tries to reposition itself after dealing with privacy scandals and the ongoing scourge of fake news spreading on the platform.

Last September, Instagram co-founders Mike Krieger and Kevin Systrom, quit after reported tensions with Zuckerberg. Jan Koum, one of the co-founders of WhatsApp, also left the company last year, months after his co-founder, Brian Acton, left to pursue other ventures. Acton has since been vocal encouraging people to delete Facebook, renewing the call as recently as Wednesday.