Facebook’s chief product officer and one of the primary architects of its signature news feed, Chris Cox, said on Thursday he was leaving the company, just days after Mark Zuckerberg revealed a plan to shift the world’s biggest social network to an encryption-focused messaging company.

Cox, one of Zuckerberg’s earliest employees and closest lieutenants, said in a blogpost he made the announcement “with great sadness” and was leaving after 13 years with the company.

“As Mark has outlined, we are turning a new page in our product direction, focused on an encrypted, interoperable, messaging network … This will be a big project and we will need leaders who are excited to see the new direction through,” Cox said in a Facebook post.

Cox’s departure portends seismic change for a company that has maintained a remarkably stable inner circle of executives throughout its 15-year history. Cox joined the company as a software engineer in 2005, served as director of human resources for a time, was promoted to chief product officer in 2014, and was elevated to overseeing Facebook’s entire “family of apps” (Facebook, Instagram, WhatsApp and Messenger) in May 2018.

Many viewed Cox as a possible successor to Zuckerberg, should the CEO ever choose to step away.

Shares of Facebook were down 1.7% in extended trading following the announcement.

In his note, Zuckerberg claimed that Cox had been considering a departure “for a few years” because of a “desire to do something else”, but had remained to help the company recover from the fallout from the 2016 presidential election. But early reports – and Cox’s own goodbye note – suggest that he was not “excited” about seeing through Zuckerberg’s plan to unite the company’s various messaging apps into a single, encrypted service.

Sources told BuzzFeed News and the New York Times that Cox’s departure stemmed from disagreements with Zuckerberg over the company’s future direction.

“Embarking on this new vision represents the start of a new chapter for us,” Zuckerberg wrote in a note to employees. “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.”

Also departing is the WhatsApp vice-president Chris Daniels, Zuckerberg said in a note to employees. Daniels also chose to resign due to concerns with Zuckerberg’s plan to integrate the messaging apps, according to the Times.

The departures demonstrate that Zuckerberg “has the courage to do what he thinks is right in the face of extremely strong dissent”, said the venture capitalist Ben Horowitz in a series of tweets reacting to the news. “He is genuinely committed to privacy in general and specifically end-to-end encryption. So much so, that he is willing to lose outstanding executives who disagree with this direction.”

Facebook does not immediately plan to fill Cox’s role in the near term, the chief executive said.

Facebook’s strategy for its family of apps has so far been led jointly by Cox and Javier Olivan, vice-president of growth.

Zuckerberg said on Thursday Olivan would now take the lead in identifying where Facebook apps should be more integrated, a key effort as the company moves to encrypt conversations on more of its messaging services and make them compatible with each other.

Cox gained oversight of WhatsApp and Instagram following the exits of their founders.

In September, the Instagram co-founders Kevin Systrom and Mike Krieger resigned as chief executive officer and chief technical officer of the photo-sharing app, owned by Facebook.

Jan Koum, the co-founder of WhatsApp, left in April last year.

Will Cathcart, vice-president of product management, will now lead WhatsApp, and Fidji Simo, head of video, games and monetization, will be the new head of the Facebook app, Zuckerberg said. Cathcart and Simo will report directly to the CEO.