Apple's latest App Store distribution numbers show that iOS 8 has now been installed on 77% of all compatible devices as of March 16, 2015.

iOS 8 Adoption Numbers

iOS 7 now stands at 20%, with all the older versions claiming a total of 3% of installs. While the new stats represent a 5% gain compared to the iOS 8 adoption numbers from February 2, 2015, they are still behind the installs of iOS 7 for close to the same time period a year ago. In early April 2014, Apple showed that iOS 7 was installed in 87% of all supported devices.

Apple launched iOS 8 in September 2014. Rival Google launched Android 5.0 Lollipop in November, but its current adoption numbers showed that it is installed on just 3.3% of all Android devices. The older Android 4.4 KitKat now has a 40.9% adoption rate.

Source: Apple