Chris Duckett of ZDNet:
Having pored over these numbers, there is a clear trend: Once a customer becomes an Android or iOS user, it is very difficult for any other mobile operating system to pry them away. A user may move between iOS and Android, but the step out to Windows Phone or BlackBerry is a rare event.
Windows Phone gains have mostly resulted from taking sales away from its closest competitors: BlackBerry and feature phone makers. Now that almost all phone sales in developed countries are one of iOS, Android, or Windows Phone, the task will be harder for Windows Phone in the future.
Chris Duckett’s article is fantastic, I highly recommend taking the time to read it all.