While many forums claim it is impossible, the community has developed several "last resort" methods to get a functional Facebook app onto these older systems: Facebook - App Store Requires iOS 15.1 or later.

AltStore is currently the most reliable way to sideload apps without "jailbreaking" your device.

If the App Store method fails, you can manually install an IPA file using a PC. Facebook Ipa For Ios 12.5.5