Facebook's app checks for the existence of the Facebook messenger app and if not found disables the messaging capabilities on your Facebook app. That's a bit shit because I have no interest in installing that invasive, bloaty app which demands access to almost every part of my phone.
Fortunately for me, I found a post on XDA forums where nemoc23 figured out that all you needed to do is create a dummy app which pretends to be "com.facebook.orca" and Facebook messaging will work again.
So, steps to install it are:
- Go to Settings > Security & Privacy
- Make sure you have "Unknown sources" enabled
- Go to Settings > Apps > Facebook
- End/terminate the app, confirm yes
- Clear the data, confirm yes (this also clears cache for you)
- Download "Messenger.apk" from XDA (footer of the first post)
- Open it and install it
- (recommended) Turn "Unknown sources" back off
- Open up the Facebook app and log back in
You'll need to go through your settings to customise notifications and stuff, but at least now you've got messaging capabilities back in the one app!