Key Facebook exec Chris Cox is leaving.

It's the biggest executive departure in years, and comes amid a broader reshuffle in the leadership team at Facebook.

On Thursday, CEO Mark Zuckerberg announced that Chris Cox — a longtime Facebook executive who most recently served as chief product officer — is leaving the company, in what is easily the most significant departure at the embattled company in years.

Cox, a key lieutenant of the 34-year-old billionaire chief exec, had been planning to leave for "a few years" but stayed on to help respond to Facebook's successive scandals and crises, Zuckerberg said in a memo to employees that the company also shared publicly.

And Fidji Simo is taking on the role of head of the Facebook app.

The heads of Facebook's various apps — Facebook, WhatsApp, Messenger, and Instagram — will now report directly to Zuckerberg, cementing the CEO's hold over them as Facebook prepares to embark on an ambitious project to partially merge the apps.

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