The Facebook apps on both Windows Phone and W10 Mobile were created by Microsoft. Those apps stopped being updated a long time ago and you could no longer find them in the store. These have now all stopped working. I was still using the Microsoft version on my L950 and it started throwing up errors this week. Now it displays a message on startup saying you need to update to the newer version.
However, if you are on W10 Mobile, there's an official app from Facebook itself (the same one that you can install on your PC) that is still supported. There's also a beta version of that same app, which gets new features first. But you don't need to use the beta version to use Facebook.