[undulose]

The FB app was developed by Microsoft while the Messenger app was developed by Facebook. Why the difference? Can't Microsoft just bring out the old FB messaging integration, like how it's done on the desktop People Hub site?
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[Piachu]

It's more complex case. With WP 8.1 and further in Windows 10 most of the functions is separated from the core of the OS. Why? Because now it's very easy to update app in case of new functionality or API Changes, what was not possible in previous OS versions. To sum it it should be better (of course quality of the integration depends on quality of the apps)