Facebook app alternatives (on Android)
There are a number of excellent replacements for the Facebook (and Messenger) apps. All are a mobile "wrapper" for the Facebook website, and include Messenger functionality. Here are the ones I've used and recommend. All are open-source:
"Face Slim" (Not on Play Store, but is on the F-Droid open-source app store):
https://f-droid.org/en/packages/org.indywidualni.fblite/ OR https://github.com/indywidualny/FaceSlim
"SlimSocial for Facebook" (Available on Play Store & F-Droid):
https://play.google.com/store/apps/details?id=it.rignanese.leo.slimfacebook OR https://f-droid.org/en/packages/it.rignanese.leo.slimfacebook/ OR https://github.com/rignaneseleo/SlimSocial-for-Facebook
and my current favorite "Frost for Facebook": (Currently has an issue where some users get locked out of their FB accounts for "security" reasons. Likely due to Frost's agent-switching. More info: https://github.com/AllanWang/Frost-for-Facebook/issues/1522)
https://f-droid.org/en/packages/com.pitchedapps.frost/ OR https://www.xda-developers.com/frost-for-facebook-fully-themeable-facebook-alternative/ OR https://github.com/AllanWang/Frost-for-Facebook