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The first challenge is getting the app installed. If you attempt to download Messenger directly from the Google Play Store on an Android 5.1 device today, you may hit a wall. Facebook frequently updates the app’s minimum API requirements, often pushing older OS versions into obsolescence.
For those wanting the full feature set, older APKs like v121.0.0.15.70 were specifically optimized for Android 5.1+ (API 22) . Step-by-Step Installation Guide Messenger Apk Android 5.1