to download. You should see a pop-up: "Download an older version of this app? The current version requires iOS 13.4 or later, but you can download the last compatible version." Select Download to confirm.

Installing the Facebook app on a device running iOS 12.5.5 (like an iPhone 6 or 5s) is a bit of a "good news, bad news" situation. While the App Store officially requires iOS 13.4 or later

Download Facebook. This "tags" the app to your account history. Sign out of their device.