For nearly six hours on Monday, Facebook Inc.’s 3.5 billion users were unable to access the company’s social media and messaging services like WhatsApp, Instagram, and Messenger due to a “faulty configuration change.” In a late-Monday blog post, the company didn’t say who made the configuration change or if it was on purpose or not. Earlier, several anonymous Facebook employees told Reuters that they believed the outage was caused by a clerical error in Facebook’s internet traffic routing. The employees claimed that the error was compounded by the failures of internal communication tools and other resources that rely on the same network to function. Insider sabotage or an accidental error, according to security experts, are both plausible scenarios. “We want to make clear at this time we believe the root cause of this outage was a faulty configuration change,” Facebook said in the blog. This is the most significant outage in the history of web monitoring service Downdetector’s records. On Sunday a whistleblower accused the social media giant of repeatedly prioritizing profit over cracking down on hate speech and misinformation. Now there’s an outage, and it’s a big one. Shares of Facebook fell by 4.9% on Monday, their biggest daily drop since last November, as users flocked to rival apps like Twitter and TikTok. This was part of a broader selloff in technology stocks. In after-hours trade, shares increased by about 0.5 percent as service was resumed. It “may take some time to get to 100 percent,” Facebook CTO Mike Schroepfer tweeted to the company’s small and large businesses, families, and individuals who depend on us. Jonathan Zittrain, director of Harvard’s Berkman Klein Center for Internet & Society, said on Twitter that Facebook had “basically locked its keys in its car”. Due to higher-than-normal usage on Monday, some users had trouble accessing their posts and direct messages on Twitter. When Instagram and Facebook go down,” Netflix shared a meme from its new hit show “Squid Game” with the caption “When Twitter and everyone is down.” The meme showed a “Twitter” holding up “everyone,” who was about to fall, in one of the most popular tweets of the day. On Facebook, one member of an ad-buying group joked that “lots of people searched today ‘how to run google ads for clients'” after the service returned. According to estimates from ad measurement firm Standard Media Index, Facebook, the world’s second-largest online ad seller after Google, lost $545,000 in U.S. ad revenue per hour during the outage. However, downtime at internet companies in the past has had little impact on revenue growth. Facebook’s services went dark at noon Eastern time, including consumer apps like Instagram, business tools it sells to companies, and internal programmes (1600 GMT). Around 5:45 p.m. ET, access began to return. As soon as the problem began, Facebook acknowledged that its apps were not working properly for some users, but it did not specify what the issue was or how many people were affected. According to Facebook’s error message, the DNS (Domain Name System) was malfunctioning, preventing users from reaching their intended destinations. In July, a similar outage at Akamai Technologies Inc (AKAM.O), a cloud services provider, brought down numerous websites. Earlier this week, Frances Haugen, a product manager on Facebook’s civic misinformation team, revealed that she was the whistleblower who provided documents to support a recent Wall Street Journal investigation and a recent U.S. Senate hearing on Instagram’s harm to teen girls. According to Reuters, Haugen was scheduled to testify before the same Senate subcommittee on Tuesday, urging the committee to regulate the company, which she plans to compare to tobacco companies that denied for decades that smoking was harmful to one’s health. Anil D’Silva, Sonya Hepinstall, Uttaresh. V and Sriraj Kalluvila edited the piece with contributions from Eva Mathews and Subrat Patnaik in Bengaluru; additional reporting came from Tiyashi Datta, Nivedita Balu, and Aakriti Bhalla in Bengaluru; Joseph Menn and Paresh Dave contributed from San Francisco; and Sheila Dang from Dallas.