Android: Facebook (in)famously forced its users to switch to the standalone Messenger app this year. What you might not have heard is the functionality is still there in the main Facebook app. Facebook Chat Re-Enabler switches it back on.
For reasons unknown, Facebook never actually removed the code for messages from its primary app. The good news is that means you can turn it back on (for now). XDA user created Facebook Chat Re-Enabler to flip the switch back.
Simply sideload the APK from the thread below and that's it. The app works in the background. The next time you start Facebook, the messages section should allow you to send messages, rather than bumping you to Messenger. Just be sure, if you have the standalone Messenger app already installed, that you remove it before installing Facebook Chat Re-Enabler.
Of course, with an app like this, there's no guarantee that it will continue to work forever. In fact there's no guarantee it will work through the rest of the week. However, the code for messages has been in the main app for several months now, so perhaps we'll have a bit of luck.
Facebook Chat Re-Enabler [XDA Developers Forum]