Here in San Francisco, almost everyone I know has iPhones. But there are a whole lotta people somewhere in the world with Android devices.
According to this article from ExtremeTech, Android shipments are up from 700,000 total in 2008, to 550,000,000 in 2012. Yes, five hundred and fifty million devices shipped worldwide running Android, with 136 million in Q3 2012 alone.
With Blackberry's penetration cut in half, and Symbian's even worse, it's now a two horse race between Android and iOS . And although in the Valley all you hear about is the iPhone, and everyone's building apps for the iPhone, a peek beyond to the rest of the world showcases the dominance of Android worldwide .
 WindowsPhone had 2% marketshare in Q3 2012, almost double from the year before. I see WindowsPhone as a dark horse competitor in this race. Microsoft has a lot of cash to figure mobile out, and they typically try to buy their way into sectors where they haven't innovated. But since Apple and Google are arch competitors to MSFT, I don't see a way for them to buy their way out of this one.
 I don't have the stats in front of me, but it's likely that while iOS' penetration is substantially lower than Android's, iOS usage on a per-device level is higher than Android's. Meaning, people are really addicted to their iPhones and iPads, and they use them a lot.