After three-and-a-half months on the market, iOS 7 is installed on some 78 percent of devices connecting to the App Store, according to the latest numbers posted on Apple's App Store support page for developers.
The company puts iOS 6 adoption at 18 percent, and older OS versions at 4 percent. The data from Apple is likely the most accurate adoption data we will see and is the most relevant for developers as its taken directly from the App Store.
Article Link: Apple's App Store Usage Numbers Peg iOS 7 Adoption at 78%