Facebook officially dropped support for iOS 12 in . This means the last compatible version of Facebook for iOS 12 was roughly v347.0.
When the prompt appears saying the current version isn't compatible, tap to install the "last compatible version" for your device. facebook ipa for ios 1255
The primary reason you are looking for an IPA (iPhone Application Archive) is that the official version of Facebook now requires iOS 13.4 or later Facebook officially dropped support for iOS 12 in