For a visual walkthrough on how to trigger the 'Last Compatible Version' download:
If Facebook is not in your purchase history, you cannot download it directly on an iOS 12 device. You must first "purchase" the app on a newer device (iPhone or iPad running iOS 13+) using your Apple ID. Once it is linked to your account, you can follow the steps above to download the legacy version on your iOS 12.5.7 device. Alternatives Facebook Lite : Check the Facebook Lite Download Facebook Ios 12-5-7
“Wish you were here, El,” her sister had written. “You haven’t replied. You haven’t been online in weeks.” For a visual walkthrough on how to trigger
Use the search bar to find Facebook in your list of previously downloaded apps. Alternatives Facebook Lite : Check the Facebook Lite
If you have never downloaded Facebook before on your Apple ID, you may need a second, newer device to "purchase" the app first so it appears in your account history.