If you are looking for an IPA (application package) to sideload, there are safer ways to get the app working without downloading potentially malicious files from third-party sites:
: You can add a shortcut to your home screen that acts like an app. While on the Facebook page in Safari, tap the icon (square with an arrow) and select Add to Home Screen Alternative: Download an "Old Version" via App Store
When the prompt appears saying the current version isn't compatible, tap to install the "last compatible version" for your device.

