This app is a result of strict co-op with Facebook Inc., as you can see (
https://play.google.com/store/apps/details?id=com.facebook.katana&hl=pl) the newest UI is introduced to make iOS, Android and Windows Phone Facebook apps similar. Probably Microsoft received a permission to use and promote their app with official Facebook logo/branding, because Facebook Inc, wasn't interested with providing their app to WP (before WP market share grown), so they just talked with MS, they reached an agreement and they are working together since the big Facebook v. 5 overhaul. Following delayed UI changes, these affecting iap messaging hub might be coming later this year. I think that no user here has appropriate knowledge to comment on FB & MS agreement, its just my argument.
Notice that Facebook's mobile website shows banner "download Facebook app for Windows Phone and browse faster" which redirects you to MS app in WP Store, so they just treat it like an official one.