Thats right, its only facebook and google internally that take advantage of XMPP while keeping the rest out. Thats what federation does and this idea is no better than jabber which we thought was a good idea 11 years ago.
This is a bad idea because a web browser should not become a chat or client as well.
You know what is a good solution to the "facebook is taking over the web" Remove their like-buttons from your pages! It is all of the web devleopers of the world who chose to add facebook scripts to their webpage that are the problem. Thats what needs to be solved. Facebook wouldnt be nearly what it is if it wasnt for all those webdevs that thought it was good idea to run fb scripts and snitch out their and the webs user.
This battle can be moved to the browser if need be, just run ghostry and better privacy by default, block all accessess to facebook domains form any other domains, and thats it.