Unlike the official Facebook app for Windows Phone, the Facebook Messenger app for Windows Phone is in fact developed by Facebook itself.
In the above video we have the Technical Lead for the app, Facebook developer Nick Reynolds share his teams experiences of developing for the Microsoft platform. He notes that there were several benefits to developing on Windows Phone including the good tools, good emulator, and the minimal level of fragmentation on the platform.
He also noted that the app was still young, and that he was working on bringing it to feature parity with their iOS and Android apps, suggesting we may see VOIP services in the future.
For the record, we note that Facebook Messenger app in fact looks about the same on iOS, Android and Windows Phone, with a pivot or tab-like structure and that none of the 3 feature any hamburger menus, which are increasingly seen as passé.