I doubt it, they had an XMPP interface, but that doesn't mean they use XMPP in the backend

I'm not sure I fully understand this comment. What does it mean to have an "XMPP backend" but not as the interface? And yes, they did use a full, open source XMPP solution for Facebook chat when it started out called "ejabberd". Ejabberd is a terrific solution if you quickly want to have a scalable, real-time messaging and event solution with low maintenance.

I'm not the person you responded to but I would assume that it would look like any other service that uses a different protocol or format on the front-end than the back-end. E.g. a REST service that talks JSON on the front end but speaks SQL to a database. So they have an XMPP backend that uses a translation layer to send a custom AJAX or socket-based protocol to the browser.

Yeah, I was pretty sure that Facebook was just using straight-up XMPP at one point in time. They have been actively trying to drive traffic to their Facebook Messenger app - first by shutting down their XMPP interface, then blocking applications that talk to their web interface, and finally by removing Facebook Messenger functionality from their mobile website.