This works if you have had Facebook installed on this Apple ID in the past.
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How to Download and Update Facebook on iOS 12.5.7: A Complete Guide download facebook ios 1257 updated
You can install the .ipa file (the "paper" file) manually if you find it on an archive site (like the Internet Archive or specialized iOS repo sites).
Before we dive into the download process, let's quickly discuss why you might want to download the Facebook app for your iOS device: This works if you have had Facebook installed
If Facebook is not in your purchase history, you cannot download it directly on an iOS 12 device. You must first "purchase" it on a newer device.
Go to > [Your Name] on the newer device and sign out of the current Apple ID. You must first "purchase" it on a newer device
If the app remains incompatible or runs too slowly, the most reliable method for iOS 12 users is to use Facebook via the Safari browser. facebook.com Share button (square with an up arrow). Add to Home Screen
Summary
Method 1: The "Purchased History" Cloud Trick (Best Solution) How To Download Facebook in iOS 9/10/11/12
If the legacy app becomes too buggy or unusable, the most reliable way to use Facebook on iOS 12.5.7 is via the web.