Facebook Beta on Windows 10 Mobile has received an update today. The update makes the app to crash constantly while trying to launch. You may need to uninstall and re-install the app to make it work. Though it may still take more than one attempt to get it working even after the re-install.
It is not clear what changes the update brings as there seem to be no visual changes. And on top of that the version seems buggy and crash-prone.