The news comes from app researcher Jane Manchun Wong, who found a rudimentary "Chats" section in the Facebook mobile app. Whereas before the Messenger button would cause you to leave Facebook and open the Messenger app, Wong discovered that Facebook appears to be testing a new "Chats" area in the app. As of now, this area only contains basic chat functionalities, and lacks reactions, call support, photo support, and more.
According to Wong, this means that Facebook will also keep the separate Messenger app around even after it adds chat functionality back into the main Facebook app, because they will "serve different markets."
All of the Facebook messaging services integration is believed to be an effort to keep people within the Facebook ecosystem and away from rival texting apps like Apple's iMessage.
Update: Facebook has provided MacRumors with the following statement: "We are testing ways to improve the messaging experience for people within the Facebook app. Messenger remains a feature-rich, stand-alone messaging app with over a billion people using it monthly to connect with the people and businesses they care about most. We do not have any additional details to share at this time."