Every so often, Apple takes data from the iOS devices that browse the App Store and publishes statistics to reflect this information on Apple's own App Store Support page. This information tells Apple how many devices are running the latest firmware version, and what other firmware versions that the company's users are operating on.
The latest sample coming to us from just this week reveals that 87% of Apple's iOS devices using the App Store are now running some version of iOS 8, leaving iOS 7 users at 12% and iOS 6 and earlier users at <2%. The numbers reveal that iOS 8 is slowly gaining traction in usage over the older iOS versions.
These numbers take into consideration everybody that uses the App Store, including new devices that are bought from stores, used devices that are bought from third-parties, devices that are upgraded (intentionally or unintentionally) over the air, and through iTunes.
iOS 8 is currently Apple's latest firmware version, with iOS 9 slowly approaching. iOS 8 has been the largest update to iOS to date with new features and APIs that developers could use to create Notification Center widgets, third-party keyboards, and more.
When iOS 9 is released to the public this Fall, it will add battery life, improvements to performance, additional security, and new features.