I would also think that it's easier to build a mobile app/social network that competes with Facebook than it is to build a mobile OS/device that competes with Android/iOS.
Hmmm. It's very easy to build a mobile platform to compete with Android -- fork it. This is Google's problem in China and with the Kindle Fire. This is probably why Google's "success" in mobile hasn't been reflected by huge stock market gains.
The success of Android forks has also impacted iOS to some hard to measure extent (were it not for Android and Android forks, Apple would, I think, be even more successful in the mobile spaces it plays in).
