After about 30 days of availability, Apple's IOS7 is on 71,75% of all iDevices according to Mixpanel. IOS6 has 24.83% of the base and earlier versions only account for ~3%.
IOS7 has been the fastest adopted IOS upgrade to date and its a clear win for Apple because it minimizes OS fragmentation making app development and support easier.
This may be why Google keep numbering its OS upgrades as minor .x upgrades (trying to bring everyone to version 4). Google has had to fight with carriers and manufacturers to make upgrades available to its users whereas Apple has a big enough stick to force everyone to play ball.