If you try to download the app directly and see an error, use the method. This only works if Facebook is already in your "purchased" history on your Apple ID. Open the App Store on your iPhone. Tap your Profile Icon in the top right corner. Go to "Purchased" (or "My Purchases"). Search for Facebook in the list.

to your Apple ID on their newer device’s App Store.

device, you can often download a compatible older version directly to your iOS 12 device: on your iOS 12 device profile icon/avatar and go to the AppleInsider Search for cloud download icon

: Download Facebook on that newer device so it’s linked to your account history. Go Back to Your iOS 12 Device : Open the App Store and tap your Profile Icon Download from History : Search for Facebook here and tap the Cloud Icon Confirm the Downgrade : A pop-up will ask: "Download an older version of this app?" Review: Facebook on iOS 12.5.5