"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," Zuckerberg wrote.

Will Cathcart will succeed Daniels as the new head of WhatsApp, Zuckerberg said. Cathcart previously ran the Facebook app, Zuckerberg said.

"Will is one of the most talented leaders at our company — always focused on solving the most important problems for people and clear-eyed about the challenges and tradeoffs we face," Zuckerberg wrote.

The company will also elevate Fidji Simo to head of the Facebook app, succeeding Cathcart in that role, Zuckerberg said. Simo has previously led the company's work on video and advertising for the Facebook app, Zuckerberg said.

"She is one of our most talented product and organizational leaders -- passionate about building community and supporting creativity, and focused on building strong teams and developing future leaders," Zuckerberg wrote.

Facebook's stock price fell more than 2 percent in after-hours trading.

Zuckerberg wrote the following:

A Note From Mark Zuckerberg

Today Mark Zuckerberg shared the following post with employees.

Hey everyone — I want to share some important updates as we organize our company to build out the privacy-focused social platform I discussed in my note last week. Embarking on this new vision represents the start of a new chapter for us.

As part of this, I'm sad to share the news that Chris Cox has decided to leave the company. Chris and I have worked closely together to build our products for more than a decade and I will always appreciate his deep empathy for the people using our services and the uplifting spirit he brings to everything he does. He has played so many central roles at Facebook — starting as an engineer on our original News Feed, building our first HR teams and helping to define our mission and values, leading our product and design teams, running the Facebook app, and most recently overseeing the strategy for our family of apps. Along the way, Chris has helped train many great leaders who are now in important roles across the company — including some who will now take on bigger roles in our new product efforts.

For a few years, Chris has been discussing with me his desire to do something else. He is one of the most talented people I know and he has the potential to do anything he wants. 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. At this point, we have made real progress on many issues and we have a clear plan for our apps, centered around making private messaging, stories and groups the foundation of the experience, including enabling encryption and interoperability across our services. As we embark on this next major chapter, Chris has decided now is the time to step back from leading these teams. I will really miss Chris, but mostly I am deeply grateful for everything he has done to build this place and serve our community.

At the same time, as we embark on this new chapter, Chris Daniels has also decided to leave the company. Chris has also done great work in many roles, including running our business development team, leading Internet.org, which has helped more than 100 million people get access to the internet, and most recently at WhatsApp, where he has helped define the business model for our messaging services going forward. 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.

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.

I'm excited that Will Cathcart will be the new head of WhatsApp. Will is one of the most talented leaders at our company — always focused on solving the most important problems for people and clear-eyed about the challenges and tradeoffs we face. Most recently he has done a great job running the Facebook app, where he has led our shift to focusing on meaningful social interactions and has significantly improved the performance and reliability of the app. In his career here, Will has helped lead our teams focused on security and integrity, and he believes deeply in providing end-to-end encryption to everyone in the world across our services.

I'm also excited that Fidji Simo will be the new head of the Facebook app. She is one of our most talented product and organizational leaders — passionate about building community and supporting creativity, and focused on building strong teams and developing future leaders. She has played key roles in building many aspects of the Facebook app, including leading our work on video and advertising. She believes deeply in helping people get more value out of the networks they've built. She has already led this team for much of last year while Will was out on parental leave, and she is the clear person to lead these efforts going forward.

Our family of apps strategy has been led jointly by Chris Cox and Javier Olivan. Chris managed the leaders of the apps directly and Javi has been responsible for all of the central product services that work across our apps, including safety and integrity, analytics, growth, and ads. Javi will now lead identifying where our apps should be more integrated. Javi is an incredibly thoughtful, strategic and analytical leader, and I'm confident this work will continue to go well. Since we have now decided on the basic direction of our family of apps for the next few years, I do not plan on immediately appointing anyone to fill Chris's role in the near term. Instead, the leaders of Facebook (Fidji Simo), Instagram (Adam Mosseri), Messenger (Stan Chudnovsky), and WhatsApp (Will Cathcart) will report directly to me, and our Chief Marketing Officer (Antonio Lucio) will report directly to Sheryl.

This is an important change as we begin the next chapter of our work building the privacy-focused social foundation for the future. I'm deeply grateful for everything Chris Cox and Chris Daniels have done here, and I'm looking forward to working with Will and Fidji in their new roles as well as everyone who will be critical to achieving this vision. We have so much important work ahead and I'm excited to continue working to give people the power to build community and bring the world closer together.