Social media services Facebook, WhatsApp and Instagram are back up and running after an outage that lasted almost six hours, Facebook says.
The company says the cause was a faulty configuration change.
All three services are owned by Facebook and could not be accessed over the web or on smartphone apps.
Downdetector, which tracks outages, said it was the largest failure it had ever seen, with 10.6 million problem reports around the world.
The services went down at about 16:00 GMT with users beginning to gain access to the sites at around 22:00.
In a statement on Tuesday, Facebook said that the faulty configuration change affected the company’s internal tools and systems which complicated attempts to resolve the problem.
Outages for major websites aren’t uncommon. There was one last week on Slack for example.
What makes this so noteworthy is the size, scale and context.
Many outages that involve a domain name system usually get resolved fairly quickly. They are often localised too, with some people unable to open a website that can be viewed in another country.
This outage, however, was global, and affected all of Facebook’s many spin-offs.
The length of time it was off grid is also unusual. There were reports of “mayhem” in Facebook headquarters, as technicians scrambled to fix the problem.
The week had already started off badly – after the whistleblower in the “Facebook Files” revelations revealed herself on Sunday.
But a bad week is fast becoming a terrible one for the social network.