Bog standard Jabber except for the changes they make - group messaging, 'seen' notifications, in line attachments, 'stickers' are all proprietary stuff which a standard jabber client doesn't support, unfortunately.

Their Facebook Messenger for Windows client was discontinued earlier this month unfortuntaely, so I'm back using Trillian and boy do I miss the group messaging feature.

(genuine question, I don't use FB chat)

How is group messaging different from MUC[1]? The naming group messaging sounds alike, but I don't know the internals of FBs feature.

in line attachments is also possible[2] - I assume you mean sending files, embedding photos in messages and such?

As for seen and stickers I don't know of a XMPP equivalent, but I am curious as to why MUC wasn't used instead of proprietary group messaging.

[1]: XEP-0045, http://xmpp.org/extensions/xep-0045.html

[2]: html and/or XEP-0231, http://xmpp.org/extensions/xep-0231.html