Apple's latest data shows that iOS 8 is now installed on 81% of devices as of April 27, 2015.
When last measured, iOS 8 adoption stood at 77%. While this shows fairly steady continued growth for Apple's latest major release, iOS 8 is still behind iOS 7's adoption rate, which was around 87% at this time in 2014.
Most of the gains for iOS 8 come out of iOS 7, which is now installed on 17% of devices. Older versions of Apple's mobile operating system, such as iOS 6, sit at 2% of installations. This data comes from Apple, using data taken from the App Store.