(AP) — Facebook along with its Instagram and WhatsApp platforms suffered a worldwide outage Monday that has extended more than three hours. Facebook’s internal systems used by employees also went down. Service has not yet been restored.

The company did not say what might be causing the outage, which began around 11:40 a.m. ET and was still unfixed more than six hours later, although scattered users have reported partial restoration of one service or another. Websites and apps often suffer outages of varying size and duration, but hours-long global disruptions are rare.

“This is epic,” said Doug Madory, director of internet analysis for Kentik Inc, a network monitoring and intelligence company. The last major internet outage, which knocked many of the world’s top websites offline in June, lasted less than an hour. The stricken content-delivery company in that case, Fastly, blamed it on a software bug triggered by a customer who changed a setting.

Facebook’s only public comment so far was a tweet in which it acknowledged that “some people are having trouble accessing (the) Facebook app” and that it was working on restoring access. Regarding the internal failures, Instagram head Adam Mosseri tweeted that it feels like a “snow day.”

But the impact was far worse for multitudes of Facebook’s nearly 3 billion users, showing just how much the world has come to rely on it and its properties — to run businesses, connect with communities of affinity, log on to multiple other websites and even to order food.

It also showed that, despite the presence of Twitter, Telegram, Signal, TikTok, Snapchat and a bevy of other platforms, nothing can truly replace the social network that has evolved in 17 years into all but critical infrastructure. Facebook’s request Monday that a revised antitrust complaint against it by the Federal Trade Commission be dismissed because it faces vigorous competition from other services seemed to ring a bit hollow.

The cause of the outage remains unclear. Madory said it appears Facebook withdrew “authoritative DNS routes” that let the rest of the internet communicate with its properties. Such routes are part of the internet’s Domain Name System, a central component of the internet that directs its traffic. Without Facebook broadcasting its routes on the public internet, apps and web addresses simple could not locate it.

So many people are reliant on Facebook, WhatsApp or Instagram as a primary mode of communication that losing access for so long can make them vulnerable to criminals taking advantage of the outage, said Rachel Tobac, a hacker and CEO of SocialProof Security.

“They don’t know how to contact the people in their lives without it,” she said. “They’re more susceptible to social engineering because they’re so desperate to communicate.” Tobac said during previous outages, some people have received emails promising to restore their social media account by clicking on a malicious link that can expose their personal data.

Jake Williams, chief technical officer of the cybersecurity firm BreachQuest, said that while foul play cannot be completely ruled out, chances were good that the outage is “an operational issue” caused by human error.

Madory said there was no sign that anyone but Facebook was responsible and discounted the possibility that another major internet player, such as a telecom company, might have inadvertently rewritten major routing tables that affect Facebook.

“No one else announced these routes,” said Madory.

Computer scientists speculated that a bug introduced by a configuration change in Facebook’s routing management system could be to blame. Colombia University computer scientist Steven Bellovin tweeted that he expected Facebook would first try an automated recovery in such a case. If that failed, it could be in for “a world of hurt” — because it would need to order manual changes at outside data centers, he added.

“What it boils down to: running a LARGE, even by Internet standards, distributed system is very hard, even for the very best,” Bellovin tweeted.

Facebook was already in the throes of a separate major crisis after whistleblower Frances Haugen, a former Facebook product manager, provided The Wall Street Journal with internal documents that exposed the company’s awareness of harms caused by its products and decisions. Haugen went public on CBS’s “60 Minutes” program Sunday and is scheduled to testify before a Senate subcommittee Tuesday.

Haugen had also anonymously filed complaints with federal law enforcement alleging Facebook’s own research shows how it magnifies hate and misinformation, leads to increased polarization and that Instagram, specifically, can harm teenage girls’ mental health.

The Journal’s stories, called “The Facebook Files,” painted a picture of a company focused on growth and its own interests over the public good. Facebook has tried to play down the research. Nick Clegg, the company’s vice president of policy and public affairs, wrote to Facebook employees in a memo Friday that “social media has had a big impact on society in recent years, and Facebook is often a place where much of this debate plays out.”

Twitter, meanwhile, chimed in from the company’s main Twitter account, posting “hello literally everyone” as jokes and memes about the Facebook outage flooded the platform. Later, as an unverified screenshot suggesting that the facebook.com address was for sale circulated, Twitter CEO Jack Dorsey tweeted, “how much?”