Facebook, WhatsApp and Instagram have been hit by an outage lasting several hours.
All three services are owned by Facebook and could not be accessed over the web or smartphone apps.
Downdetector, which tracks outages, logged tens of thousands of outage reports for all three platforms from around the world.
Facebook’s chief technology officer Mike Schroepfer has tweeted his “sincere apologies” to those affected.
“We are experiencing networking issues and teams are working as fast as possible to debug and restore as fast as possible,” he wrote.
Some people also reported problems using Facebook’s virtual reality headset platform, Oculus, and apps which require Facebook logins were affected, including Pokémon Go.
An outage of this scale for such a long time is rare.
Another disruption in 2019 left Facebook and its other apps mostly inaccessible across the world for more than 14 hours.
As yet there has been no official reason given for the problem. But online network experts speculated it may involve an error with DNS, or the domain name system, for Facebook sites.
DNS is often compared to an address book or phone book for the internet, pointing web browsers to the computer system which serves the website they are looking for.
Several other tech companies, including Reddit and Twitter, have poked fun at the social media giant’s predicament – prompting responses from the affected apps.
The disruption comes the day after an interview with a former Facebook employee who leaked documents about the company.
On October 3, Frances Haugen told CBS news that Facebook had prioritized “growth over safety”.
On October 5, Frances Haugen will testify before a Senate subcommittee in a hearing titled “Protecting Kids Online”, about the company’s research into Instagram’s effect on the mental health of young users.