Matrix has a bridge that interfaces (very well might I add) with messenger. I haven't looked at the code but some control messages suggest the bridge uses a MQTT listener to connect to messenger.
Fun fact: there is a 'wishlist`-priority task sitting in FB's Tasks system with detailed plans for how to change the MQTT API to break third-party clients should any of them become too popular.
There's nothing I can cite. Saw it myself many years ago. I doubt many of the people who worked on that iteration of Messenger are even there any more, but it shows the attitude of the organization as a whole.
