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One of Facebook’s top executives and a close adviser to CEO Mark Zuckerberg said on Thursday he’s leaving the company.

Chris Cox joined Facebook in 2005, the year after its founding, making him one of the company’s longest-tenured employees. Since 2014 he’s been chief product officer, a senior executive role that most recently gave him oversight of all the company’s apps including Facebook and Instagram.

Cox, in a post on Facebook, did not give a reason for leaving. He thanked Zuckerberg and other executives and said he was proud of their work.

“Social media’s history is not yet written, and its effects are not neutral,” Cox wrote. “It is tied up in the richness and complexity of social life. As its builders we must endeavor to understand its impact — all the good, and all the bad — and take up the daily work of bending it towards the positive, and towards the good. This is our greatest responsibility.”

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Cox’s long tenure and high-level role led to speculation in the media that he could one day succeed Zuckerberg. Wired magazine last year called him “the new Mark Zuckerberg.”

Zuckerberg, in a separate memo posted to Facebook's corporate website, said that, “for a few years, Chris has been discussing with me his desire to do something else.”

It was not clear what Cox would do next, and Zuckerberg said he was purposefully not naming someone to take on the full range of Cox’s responsibilities.

“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,” Zuckerberg wrote.

Another Facebook executive, the chief of messaging service WhatsApp, will also leave the company, according to the memo. Chris Daniels became the head of the service less than a year ago in a management shake-up after Jan Koum, one of WhatsApp’s co-founders, said he would leave.

Facebook posted Zuckerberg’s memo as the stock market was closing for the day. Shares fell 2 percent in after-hours trading.

The past two years have seen a series of executives depart Facebook as scandals have repeatedly rocked the company. Those leaving for various reasons have included the company’s general counsel, chief security officer, policy and communications chief, the co-founders of WhatsApp and Instagram and a co-founder of virtual-reality unit Oculus.

Zuckerberg last week released a 3,200-word memo outlining what he called a new vision for the company and its various apps, with less of an emphasis on public social media posts and a higher priority on private messaging protected by encryption.

In search of revenue growth, the company is testing various possibilities such as ways to send money to other people, a cryptocurrency, and more closely knitting together apps such as Instagram, WhatsApp and Messenger.