It is highly likely you are looking for a Facebook IPA file compatible with iOS 12.5 (often typoed as 1255) to install on an older iPhone or iPad.
Since you cannot simply download the current version of Facebook, you need to find a specific IPA version compatible with iOS 12. facebook ipa for ios 1255
Frequently Asked Questions
Are you trying to install the app on a specific device like an iPhone 6, or are you having trouble with an "Unable to Purchase" error? Facebook - App Store Requires iOS 15.1 or later. Install Facebook on your Apple iPhone XR iOS 12.0 - Telstra It is highly likely you are looking for
Functionality