generally only offers the latest version (Android 6.0+), you must use trusted APK repositories for legacy files: Alibaba.com Older versions of Facebook Lite (Android) | Uptodown
Use the direct link search on APKMirror for "facebook-71-0-0-20-105-android-apk-download". Keep a backup copy of the APK on your SD card.
It is designed for low-memory, slow-network environments, often compatible with Android 2.2 and above.
This version consumes the least background resources, works flawlessly on 32-bit processors, and includes all core features: News Feed, Messenger integration, notifications, and story uploads.
