We reported earlier this month that Twitter was no longer functioning on Windows Phone 8.1 devices, and it seems the same issue has spread to Facebook.
Windows 8.1 and Windows Phone 8.1 users who attempt to log in either to Facebook or Facebook Messenger find themselves locked out of the app as the firm no longer allows users to login with those apps.
This is not a new development, Facebook had previously announced that they would be shutting off support for their Windows Phone 8.1 apps, and now they have.
For the few users who want to remain using Facebook on Windows 8.1, they are welcome to upgrade to Windows 10 Mobile, where Facebook remains semi-actively supported. Barring that, iOS or Android.