If all you do is use Facebook on your phone then I guess this is magnificent.
I wouldn't be surprised if this is FB's first step into building an own mobile OS (or own distribution) since the main task of a mobile device is still communication with close contacts -- and that's all Facebook is about.
All Facebook should need is a browser for their OS, if they provide a browser as good as or better than Chrome for Android they should be fine. The Facebook apps we use on desktop are just browser apps, and it'd be very easy to win developers over that way, plus the App Center is already there serving web apps.
And going with the web should help them stay away from competing with partners like Apple and Microsoft.
All in all their bottom line is Facebook as a service, all they need is to provide something that brings users to using Facebook more and to see more ads.
They've likely even thought of this themselves but are waiting it out to see the web mature more.