https://techcrunch.com/2019/04/12/facebook-mobile-app-chat/

Facebook upset millions upon millions of users five years ago when it removed chat from its core mobile app and forced them to download Messenger to communicate privately with friends. Now it looks like it might be able to restore the option inside the Facebook app.

That’s according to a discovery from researcher Jane Manchun Wong who discovered an unreleased feature that brings limited chat features back into the core social networking app. Wong’s finding suggests that, at this point, calling, photo sharing and reactions won’t be supported inside the Facebook app chat feature, but it remains to be seen if that is simply because it is currently in development.