Microsoft has apparently pulled the recently-released Facebook app made for Windows 10 Mobile. The 10.1 version of the app was available to download earlier this week from the Windows Store, but it has now been replaced with the old 184.108.40.206 version that also works with Windows Phone 8.1
It's more than likely that Microsoft jumped the gun a bit on releasing the Facebook app for Windows 10 Mobile. If you still have the 10.1 version installed on your phone, you may want to keep it for a while as users are reporting that if they delete and then redownload the app they now get the older 220.127.116.11 version.
Thanks to Bagsy for the tip!