Social media services 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 nearly 80,000 reports for WhatsApp and more than 50,000 for Facebook.
Data from the website suggests the outage is affecting user accounts across the world.
Some people also reported problems using Facebook’s virtual reality headset platform, Oculus.
And apps which require Facebook logins were affected, including Pokémon Go.
Facebook said: “We are aware some people are having trouble accessing our apps and products. We are working to get things back to normal as quickly as possible and apologise for any inconvenience.”
No official reason for the problem was given.
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.
Previous issues with DNS led to widespread outages of several major sites earlier this year.
In one of those instances, it emerged that the blackout was caused by a single customer of a widely-used service who changed their settings, triggering a software bug affecting a huge number of websites.
It is rare for such problems to affect a single tech giant such as Facebook, or for the issue to remain unfixed for a long time.
Several other tech companies, including Reddit and Twitter, have poked fun at the social media giant’s predicament – prompting responses from the affected apps.