In the recent screen shots of Windows Phone 8.1 GDR1 published by Microsoft the company may have been hiding a secret in plain sight.
Vietnamese site Techrum.vn note Microsoft appears to be showing Chat on Facebook in the pictures, a feature which has been absent since the 8.1 update.
Facebook has recently moved the Chat feature out of the Facebook app into a dedicated Messenger app, to enable it to monetize it separately. It seems unlikely that they will allow Microsoft to move it back into the OS again, but then the pictures above speak a 1000 words.