If you can't find a previous version linked to your iTunes account from one of the backups you'll have to do it the "Shady" way, download appsync, and look for the ipa version number on Google and you'll find it. I don't recommend this route because AppSync is a piracy tweak and also known to cause complexities with other tweaks, try to wait for an update or clear your cache with iCleaner and reinstall. if the iCleaner method fails, try booting in SafeMode and launch Facebook app see what happens.
I had/have the same problem. Deleted, respring, reboot, icleaner, reinstall...several times....facebook still crashes.
I downloaded an app called social duplicator that came out recently. Made another copy of facebook, and the new copy works great. The old one still wont work, so i just put it in a folder and hid it away.
It also updates, the new facebook app created i mean...