Side note: I've got both iOS and Android devices, and nearly every app on Android feels like a cheap knockoff of the iOS equivalent. I can't think of a single situation where I prefer the Android app over the iOS app.
For a free app with an international audience like Facebook, this seems like the wrong strategy
I was speaking to a friend recently about writing iOS & Android apps and their Android apps literally take 4x longer than their iOS apps to build because of hardware fragmentation. As in, bugs in the hardware of phones like the SGS3 (still a very large portion of the market) causing them to have to implement massive workarounds and complete full testing on actual hardware devices.
It is around 40%-50% of the developed world. Japan is the #1 iPhone nation at around 55% share, the US alone I remember reading its around 52% of smartphones are iPhones. Also 50% of iPhones out there use the latest OS (a month from release) and that will only accelerate while only 25% of Androids use the latest OS.
Edit: Also Apple is betting on turning China into an iPhone nation. There were around 20 million pre-orders for the iPhone 6 in China. The iPhone marketshare might shrink as more people get smartphones, but the iPhone isn't going anywhere soon, it seems its going to be the phone of the middle class around the world.