The Facebook APK file is the installation package for the Facebook app on Android devices. The APK file contains all the necessary files and data to install and run the app on an Android device.
Users search for archived versions (like Facebook Lite 1.x or 2.x) on community-trusted repositories. Enabling Unknown Sources: Android 4.4.2 requires users to go into Settings > Security Facebook Apk File For Android 4.4.2