Like its Android and iOS cousins, Facebook Messenger allows you to share messages with your Facebook friends without opening the dedicated Facebook app. It should also incorporate a number of other features that the app is able to do.
One thing we are not sure will be featured is the chat head function. We will not rule it out since it is usable on iOS. Also not known is if the app will enable free calls in the same way it can elsewhere. Either way, the benefit to a dedicated first-party app (that is, developed by Facebook), particularly Facebook, is that the app will get updated regularly.
How this will impact the deep integration of Facebook with the Windows Phone platform as a whole is yet to be seen.
source and photo credit: WMPoweruser