It seems like with such a strong engineering team, Facebook should have really built their own phone. It's impressive that they re-wrote a lot of the major components the iOS development process and optimized them for their scale. I think a Facebook style development platform on it's own hardware would be an interesting contrast to Google's and Apple's solutions. One challenge with learning this type of development stack is that you're almost not even learning traditional iOS development so your skills aren't truly transferrable.