Every time I use the Facebook iPhone app to message, it prompts me to use the iOS Messenger app, and then takes me there. Every. Single. Time.
Messenger via a desktop browser now relegates you to this tiny little box in the lower right corner of my 27" screen that i can barely see. I have to hit options, see full conversation to get a reasonable messaging view.
I would have assumed with the WhatsApp purchase messaging strategy would be a priority...but my experiences combined with the above article makes me seriously wonder what it is...