Facebook CEO Mark Zuckerberg is turning to an old friend and former Harvard teaching assistant, Andrew “Boz” Bosworth, in a time of trouble for the company.

Last week, a damaging series of reports in The Wall Street Journal showed major problems in the company’s ecosystem, including a lack of content moderators for markets outside the U.S., an avalanche of anti-vaccine misinformation in user comments, and Facebook-owned Instagram’s negative effect on teens’ mental health.

Some of the reports said Facebook employees and execs knew of these problems but could not or would not fix them. Lawmakers have already pledged to question execs from Facebook and other Big Tech companies over social media’s effects on teens.

On Wednesday, Facebook shuffled its leadership. Mike Schroepfer, its CTO of more than eight years, will resign next year and will be replaced by Bosworth.

It’s not clear why Schroepfer is leaving, or whether it has anything to do with the Journal reports. In his note announcing his resignation, he said he hoped to dedicate more time to family and philanthropy while still helping out with recruiting and with artificial intelligence technologies as the company’s first senior fellow.

With Bosworth, Zuckerberg is once again turning to one of his most trusted deputies.

Since joining in 2006, Bosworth has gained a reputation as Zuckerberg’s go-to-fix-it guy. He has developed key products and turned around crucial divisions, including hardware and Facebook’s bread and butter: advertising. He has a reputation for being direct with his peers and subordinates. He also frequently posts his thoughts on technology, leadership and personal growth — internally and on his public blog.

Some of these thoughts are unusually blunt for a corporate exec. For instance, in a leaked memo from January 2020, Bosworth said Facebook was more like sugar than a toxin.

“While Facebook may not be nicotine I think it is probably like sugar,” he wrote. “Sugar is delicious and for most of us there is a special place for it in our lives. But like all things it benefits from moderation.”

In a 2016 memo that leaked, he wrote about an attitude among some Facebook employees that connecting people is “de facto good” even if it sometimes leads to bad outcomes, like bullying or a “terrorist attack coordinated on our tools.” After the leak, Bosworth and Zuckerberg explained that the memo was meant to criticize this mindset among Facebook employees rather than defend it.

Bosworth is also one of Facebook’s most accessible executives, posting frequently on Twitter or holding Q&A sessions on Instagram. Most recently, he launched a podcast called “Boz To The Future” where he and guests discuss the latest in technology.

He is a polarizing figure within the company as well. One former employee who spoke on condition of anonymity so as to not break is non-disclosure agreement with Facebook told CNBC that Bosworth thinks he’s a genius, but probably just got lucky in his career. However, a former company executive who worked directly with Bosworth for several years told CNBC that Bosworth is a passionate leader to work for who demands greatness out of his employees.

Facebook declined to comment.

Bosworth met Zuckerberg at Harvard as a teaching assistant in an artificial intelligence class. After Zuckerberg founded Facebook in 2004, Bosworth joined the company in January 2006 as one of the company’s earliest software engineers.

Within months, Bosworth had left his mark. He was one of the few software engineers who built what is now the most significant Facebook feature, News Feed. Prior to News Feed’s launch in September 2006, Facebook was a bunch of profiles users could jump between, leaving posts on each other’s “walls” as desired. News Feed brought all of these posts together in a single, never-ending screen, where the content just kept coming. Bosworth is regarded as the godfather of News Feed, a former executive told CNBC.

Some Facebook users were initially upset that their messages to one another were now easily visible for all their friends to see. But the feature eventually became a hit.

As Facebook transitioned from being primarily web-focused to mobile-first in 2012, Zuckerberg tapped Bosworth to lead the development of the company’s advertising products. In that role, Bosworth took a dysfunctional hodge-podge of products in a division that had been struggling, the former Facebook executive told CNBC, and he turned it into a a nearly $27 billion money-maker by the end of 2016.

In August 2017, Facebook announced that Bosworth would manage consumer hardware, including the company’s struggling skunkworks division of Building 8.

Even though Bosworth had no experience working on hardware, Zuckerberg turned to him to fix the teams, which included the virtual reality division Oculus acquired in 2014 for $2.3 billion. Oculus had barely released its first consumer headset, the Rift, a year earlier with little consumer success, and Building 8 was struggling to deliver products at the overzealous pace Facebook was expecting.

Over the past four years, Bosworth has reorganized and refocused Facebook’s hardware unit, which is now called Facebook Reality Labs.

Now, the company finally has a broad stable of hardware gadgets available for purchase. These include the Oculus Quest headset, the Portal, Portal Go, Portal+ and Portal TV video-calling devices, and smart glasses built in conjunction with Luxottica called Ray-Ban Stories. Earlier this year, Facebook also announced a new team within Reality Labs that will focus on the metaverse — a future space in virtual reality where people can meet.

Facebook has yet to break out specific sales figures for its hardware devices, but the company’s other revenue category, which includes Facebook’s Workplace enterprise software division, has grown to nearly $1.8 billion in 2020, up nearly 118% from $825 million in 2018.

Now, with a key spot needing to be filled, Zuck is turning to Bosworth again.