I do my FB messaging through Messages on my Mac and the FB Messages app on my phone. Works great.
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.
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
Maybe you'll like it :-)