Following an hours-long outage Monday, Mark Zuckerberg, co-founder and CEO of social media platform Facebook, lost $6 billion in net value.

The outage, which began at lunchtime Monday, knocked out Facebook, WhatsApp, and Instagram for six hours before being restored about 6.30 p.m. ET.

“We are aware that some individuals are experiencing difficulties accessing our applications and products,” Facebook stated on its official Twitter account. “We’re working hard to restore normalcy as soon as possible, and we regret any inconvenience.”

Because of the outage, Facebook shares fell 5%, causing Zuckerberg, who owns 15% of the firm, to lose $6 billion in personal fortune.

Because of the interruption, Zuckerberg fell from fifth to sixth place on Forbes’ real-time billionaire’s list. As of 9 p.m. ET Monday, Zuckerberg had a net worth of $116.8 billion, trailing Larry Ellison, who has a net worth of $117.5 billion, Bill Gates, who has a net worth of $128.8 billion, Bernard Arnault, who has a net worth of $173.3 billion, Jeff Bezos, who has a net worth of $186.6 billion, and Elon Musk, who has a net worth of $201.2 billion.

While Facebook has yet to provide an explanation for the outage, several security experts believe it was caused by a fault with the platform’s Domain Name System (DNS).

“ThousandEyes testing reveal that the Facebook application became unavailable at 15:40 UTC on October 4 due to DNS failure. At the time, Facebook’s authoritative DNS nameservers became unavailable. “As of 17:02 UTC, the incident is still ongoing,” ThousandEyes, Cisco’s internet analysis subsidiary, tweeted on its Twitter account.

The massive disruption happened after “60 Minutes” aired a program in which Frances Haugen, a former Facebook employee turned whistleblower, stated that the site is aware that it is being used to disseminate hatred and misinformation.

“The thing I saw at Facebook over and over again was there were conflicts of interest between what was good for the public and what was good for Facebook. And Facebook, over and over again, chose to optimize for its own interests, like making more money,” Haugen said, according to CBS News.

“Facebook has realized that if they change the algorithm to be safer, people will spend less time on the site, they’ll click on less ads, they’ll make less money.”

Haugen is scheduled to testify before a Senate subcommittee Tuesday about the social media platform’s impact on the youth.