Social media services Facebook, WhatsApp and Instagram suffered an outage that lasted for at least six hours on Monday, Facebook said.
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.
It added that there was “no evidence that user data was compromised as a result of this downtime”.
On Monday, Facebook tweeted its apologies to those affected by the outage.
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. A disruption in 2019 left Facebook and its other apps mostly inaccessible across the world for more than 14 hours.
The disruption comes the day after an interview with a former Facebook employee who leaked documents about the company.
Frances Haugen told CBS news that the company had prioritised “growth over safety”.
On Tuesday she 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.