[Hardware] Additional reporting by CNBC’s Samantha Subin.

[Angel of Death]

Facebook has apologized for the mass outage that left billions of users unable to access Facebook, Instagram, WhatsApp and Messenger for several hours. “To all the people and businesses around the world who depend on us, we are sorry for the inconvenience caused by today’s outage across our platforms,” said Santosh Janardhan, Facebook’s vice president of infrastructure, in a blogpost late Monday. WATCH NOW VIDEO06:11 Facebook Under Fire: What steps need to be taken, and what level of regulation is the company willing to accept? The outage, which prevented users from refreshing their feeds or sending messages, was caused by “configuration changes on the backbone routers,” Janardhan said, without specifying exactly what the changes were. The changes caused “issues” that interrupted the flow of traffic between routers in Facebook’s data centers around the world, he added. “This disruption to network traffic had a cascading effect on the way our data centers communicate, bringing our services to a halt,” Janardhan said. Facebook, Instagram and WhatsApp stopped working shortly before noon ET, when the websites and apps for Facebook’s services were responding with server errors. Just after 7 p.m. ET, around six hours after the platforms went offline, Facebook CEO Mark Zuckerberg wrote on his Facebook page: “Facebook, Instagram, WhatsApp and Messenger are coming back online now.” He added, “Sorry for the disruption today – I know how much you rely on our services to stay connected with the people you care about.” The outage marked the longest stretch of downtime for Facebook since 2008, when a bug knocked the site offline for about a day, affecting about 80 million users. The platform currently has around 3 billion users. WATCH NOW VIDEO02:57 Facebook sells off amid whistleblower allegations and a widespread user outage In 2019, a similar outage lasted about an hour. Facebook blamed a server configuration change for that incident. The outage came one day after the whistleblower who leaked private internal research to both The Wall Street Journal and Congress revealed herself ahead of an interview with the CBS program “60 Minutes.” The documents, first reported in a series of Journal stories, revealed that the company’s executives understood the negative impacts of Instagram among younger users and that Facebook’s algorithm enabled the spread of misinformation, among other things. Facebook shares closed down almost 5% on Monday but they were up modestly nearly 1% in early trading on Tuesday.