Since you cannot simply download the current version of Facebook, you need to find a specific IPA version compatible with iOS 12.

I should also check if 125.5 is actually a version that exists. Sometimes people confuse the build number with the version. The current version of Facebook as of last check was 304.something. Wait, that's way higher. Maybe 125.5 is an old version. If it's an older version, some iOS versions might not support newer builds, so they might be stuck with an older version.

Also, possible issues with the update, like crashes, might prompt the user to seek a specific version. If 125.5 was unstable, they might want to go back. But Apple doesn't allow installing older versions unless via backup. So maybe the user heard about 125.5 from somewhere specific, like a beta program.

The search for a typically involves users trying to run the app on older Apple devices—like the iPhone 5s, iPhone 6, or iPad Air—that cannot update past iOS 12. Because the official Facebook App Store page now requires iOS 15.1 or later , the standard "Get" button may not work for a fresh install. How to Install Facebook on iOS 12.5.5

If you have previously downloaded Facebook on your Apple ID, you may be able to get a compatible version. Tap on your Profile icon (top right) and go to Search for "Facebook." Cloud download icon