:If you haven't "purchased" Facebook before, sign in to your Apple ID on a newer device (running iOS 13+) and download Facebook there. This adds the app to your "Purchased" history, allowing you to follow the steps above on your iOS 12 device.
Before you hunt down a sketchy IPA file, try the legitimate method: facebook ipa for ios 1255
Even if you find an IPA, you need a way to install it (using Sideloadly, AltStore, or a jailbreak). Unless your device is jailbroken, free Apple Developer accounts require re-signing the app every 7 days. Enterprise certificates (used by sideloading stores) are revoked by Apple constantly. :If you haven't "purchased" Facebook before, sign in