Social networks Facebook and Instagram, as well as messaging service WhatsApp, were unavailable on Sunday for more than three hours, users said.
The website Down Detector reported that thousands of people globally had complained about the Facebook-owned trio being down from 11.30 BST onwards.
Facebook users were presented with the message: “Something went wrong.”
At 14:50, the site said it had resolved the issue after some users “experienced trouble connecting” to the apps.
A spokesman for the company added: “We’re sorry for any inconvenience.”
Facebook did not comment on the cause of the problem, or say how many users had been affected.
In March, Facebook experienced one of its longest ever outages, with some users around the globe unable to access its site, as well as Instagram and WhatsApp, for more than 24 hours.