I've noticed in the last few days that Facebook has made some sort of backend changes and now recognizes WP's built in chat the same way it does the messenger apps on ios and android. From the website, my friends with windows phones (both 7 and 8) now show the little mobile icon instead of simply always being online. When I got my 8X the other day it also now says "[friend] is on mobile" in the message thread. I don't know if that's an 8 specific thing or if they just happened to flip the switch on it the same day as I upgraded so it may work on 7 as well.