Recently Facebook announced Facebook Home to be the leader of the mobile space (attracting most eyeballs for the longest time). The first Facebook phone rumours came out around a couple years ago and at that time there were not many who would agree to this idea and also who were optimistic of the success of this phone.
With Facebook Home, the company has essentially co-opted Google's Android platform by building a launcher or a wrapper on top of Android that controls all the user interactions. There are various reasons why Facebook may win the mobile war in coming years over the existing companies and competitors Apple, Google and Microsoft. Some of them can be:
Everything they will get from you using Facebook Search on your device.
Every SMS and message is going through Facebook. As a result they can sense when you're fighting with your spouse and this would be just the right time to inject some florist ads into the cover feed Facebook already knows who you are connected to through their site. Now they'll also know how many times one has called, texted, or otherwise interacted with any person irrespective of whether they are on Facebook or not.
They will come to know which businesses' pages have been "Liked", they make it easier to utilize Facebook events for your main calendar/appointment book, that unlocks even more data for them to utilize.
Looks like Facebook has decided to give some serious competitions and if the idea works they will be among the top positions in the mobile segment as well!