Facebook has stopped working, with Android users in particular reporting major problems opening the app.
The issue appears to have arrived after a new update was pushed out to users through the Google Play Store.
There were hundreds of reports of issues, with problems spread across the world, according to the website Down Detector.
Many users saw a message telling them the app hadn't properly loaded, and that they should try again. But trying again doesn't actually seem to fix the problem, which appears to be caused by an issue at Facebook.
Others said that they could not even reach that stage, and that the app just crashed before it actually loaded up.
The new update appears to have arrived today. Because Android phones are usually set to auto-update in the background, many users might not even be aware that they have the newest version of the app.
Sometimes, completely shutting or even deleting and then re-installing problem apps can force them to start working again. But the Facebook problems do not appear to be solved by that workaround.