Here is a comprehensive guide to understanding your options, finding the correct file, and ensuring a smooth installation. Why Use an APK for Android 5.1?
Android 5.1 Lollipop was released in 2015. Over time, the may stop showing the latest version of Facebook for older devices because the hardware can no longer support the newest features. By downloading an APK (Android Package Kit) directly, you can manually install a version of the app that is compatible with your specific system architecture. Choosing the Right Version: Facebook vs. Facebook Lite
Go to Settings > Security and toggle on "Unknown Sources." This allows you to install apps from outside the Play Store.
Once you have downloaded the correct APK file to your phone's storage, follow these steps to install it:
This usually means you tried to install a version that is too new for Android 5.1. Delete the file and look for an older "Legacy" version or the Lite version.