Rob Leathern, one of Facebook’s fiercest advocates for its ad business, left the company over the New Year, after spending four years as the head of business integrity.

Leathern made the announcement about leaving Facebook on New Year’s Day, although he has not yet disclosed where he will land next. Facebook declined to comment on his departure, but his exit appears to be on amicable terms with the company. Since 2017, Leathern has been one of the most visible people defending Facebook and its policies publicly. Leathern was active on Twitter, where he often scrapped with reporters and commented on issues related to the social network and the ad business.

Leathern indicated that his next role will keep him close to the types of issues he handled at Facebook, including topics that cover privacy, data and technology. He promised to update people about his next job through a Substack newsletter.

It’s fitting that Leathern tweeted about his move in a 12-part Twitter thread on Friday. “Working on Business/Ads Integrity/Trust at Facebook has been my most rewarding professional experience,” Leathern tweeted. “While it’s been difficult and demanding work at times, the passionate & dedicated crew I worked with have accomplished a great deal in these last few years!”

“What's next? While I'm not going to be working on ads directly, it will be a part of what I work on as I will be staying in the tech/data/privacy space. I will share more about where I’m going in the next week or two,” Leathern said.

Leathern declined to comment further when reached for comment on Monday.

Facebook has undergone changes in its ranks in recent months and the company faces new challenges in the year ahead. In September, Facebook named Alex Schultz as its new chief marketing officer following the departure of Antonio Lucio.

Last year, Facebook was criticized along with other giants in digital advertising for enabling abuses in areas like political advertising. Leathern was one of the key figures publicly advocating for Facebook, explaining how it was fixing the political ads process to crack down on bad actors while also enabling candidates, campaigns and political issue advertisers to continue using the service.

Facebook had public disputes with major politicians, including President-elect Joe Biden’s campaign, which criticized the social network’s procedures for handling misinformation in campaign ads.

This year already brings more scrutiny for the social network and advertising. Facebook is the subject of a federal anti-trust investigation, and dozens of states are probing the digital advertising market, including Facebook and Google’s potential roles in controlling parts of the machinery.

Facebook is losing one of its loudest proponents in Leathern just as it needs more allies to help present its case to the public and business world. Some of Facebook’s advertisers were lamenting the loss of Leathern on Twitter over the weekend, too. Joshua Lowcock, chief digital officer at UM Worldwide, said: “You have always been really responsive when issues are flagged both privately and publicly here on Twitter.”