Facebook, and its Instagram and WhatsApp platforms, were gradually recovering from a global outage that started Monday evening.

Facebook said late Monday that it’s been working to restore access to its services and is “happy to report they are coming back online now.” The company apologized and thanked its users for bearing with it. But fixing it wasn’t as simple as flipping a proverbial switch. For some users, WhatsApp was working for a time, then not. For others, Instagram was working but not Facebook, and so on.

Facebook did not say what might have caused the outage, which began around 11:40 a.m. ET and was still not fixed more than six hours later.

“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.

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

Mike Schroepfer, Facebook’s outgoing chief technology officer, later tweeted “sincere apologies” to everyone impacted by the outage. He blamed “networking issues” and said teams are “working as fast as possible to debug and restore as fast as possible.”

There was no evidence as of Monday afternoon that malicious activity was involved. Matthew Prince, CEO of the internet infrastructure provider Cloudflare, tweeted that “nothing we’re seeing related to the Facebook services outage suggests it was an attack.” Prince said the most likely explanation was that Facebook mistakenly knocked itself off the internet during maintenance.

Facebook did not respond to messages for comment about the attack or the possibility of malicious activity.

While much of Facebook’s workforce is still working remotely, there were reports that employees at work on the company’s Menlo Park, California, campus had trouble entering buildings because the outage had rendered their security badges useless.

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 online communities, log on to multiple other websites and even order food.

It also showed that despite the presence of Twitter, Telegram, Signal, TikTok, Snapchat and a bevy of other platforms, nothing can easily replace the social network that over the past 17 years has effectively evolved into critical infrastructure. The outage came the same day Facebook asked a federal judge that that a revised antitrust complaint against it by the Federal Trade Commission be dismissed because it faces vigorous competition from other services.

There are certainly other online services for posting selfies, connecting with fans or reaching out to elected officials, But those who rely on Facebook to run their business or communicate with friends and family in far-flung places saw this as little consolation.