It boils down to this: Not including iPhones, even the most popular smartphones only sell about 10 to 20 million units.
Meanwhile, Facebook has more than 1 billion active users. Why only give a tiny percentage of those users the best Facebook mobile experience?
Or as Zuckerberg told Steven Levy of Wired:
If we did build a phone, we’d only reach 1 or 2 percent of our users. That doesn’t do anything awesome for us. We wanted to turn as many phones as possible into “Facebook phones.” That’s what Facebook Home is.
So Facebook's strategy really is to make every phone a Facebook phone. (It'll only work on six devices at first.) Meanwhile, Facebook already deeply integrated in the iPhone and there are a suite of Facebook-branded apps that let you do cool stuff like share photos (Instagram) and make free phone calls over WiFi (Messenger).
It's probably the smartest move for Facebook right now. It'd be a longshot to successfully tweak Android to the point where it creates it's own mobile ecosystem, especially when there are so many other platforms for people to choose from. There would be little incentive for people to choose a true "Facebook Phone" over the established platforms.
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