Social media services Facebook, WhatsApp and Instagram were on Monday hit by a worldwide outage.
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 users of Facebook services across the world.
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.