Social media services Facebook, WhatsApp and Instagram are back and running after an outage that lasted almost six hours, Facebook says.
It blamed an internal technical problem that affected Facebook’s services and employees reporting work cards and emails.
Services were down from about 16:00 GMT to lasted about 22:00 on Monday.
But the company stated there was “no indication that user data was agreed”.
Shera Frenkel, the New York Times technology reporter, told the Today program part of the reason it took so much time to fix was due to “people trying to figure out what this problem was could even physically get into the building” to work out what went wrong.
In a statement, Facebook said that the wrong configuration change affected the company’s internal tools and systems which had complicated attempts to solve the problem.
The downfall detector, which tracks outages, reported some 10.6 million problem reports worldwide. However, the real number of people affected is much higher: more than 3.5 billion people use Facebook, Messenger, Instagram and Whatsapp.
Many have cut themselves off from family and friends with whom they interact across the various services, while small businesses that use social media to connect with customers have been confronted with the prospect of an unexpected financial hit.
According to business website Fortune, it also estimated Facebook founder Mark Zuckerberg $ 6 billion (£ 4.4 billion) at a point when shares fell. Zuckerberg apologized for the inconvenience.