Facebook has stopped working properly for some users.
People are seeing errors blaming "maintenance" and are unable to post, comment or upload.
The outage came at the same time as problems at Facebook-owned Instagram, where users were also unable to upload photos or update their stories.
Most reports were coming from the US and Europe, according to tracking website Downdetector. But issues appeared to be being seen globally, with vast numbers of users across reporting problems.
The issues did not appear to be hitting every single user of Facebook, with some able to log in as normal.
They also did not appear to be focusing on every part of the site. Some reported that they were able to load up their news feed or other parts of Facebook as normal – but that trying to upload a photo, comment on another person's post or use other functionality did not work properly.
Facebook has repeatedly been hit by a number of outages this year. One of them, in March, lasted for more than a day and took down Instagram and WhatsApp as well as the main site.
Then, it said there had been a "server configuration change" in a short statement explaining the more than 14-hour outage.
"Yesterday, as a result of a server configuration change, many people had trouble accessing our apps and services," it wrote in a tweet. "We've now resolved the issues and our systems are recovering. We’re very sorry for the inconvenience and appreciate everyone’s patience."
But they have gone largely unexplained, and Facebook has not said why it has been hit by so many problems.
