According to Apple, iOS 8 is now running on 73% of iOS devices. This is a small 1 percentage point higher than it was two weeks ago. The data also notes 24% of iOS devices are running iOS 7, and 3% running older firmware. Keep in mind, these are devices that are capable of connecting to the App Store.
Analytics from MixPanel shows similar data, reporting that 75.74% of compatible devices.
To put this into perspective, iOS 7 had an 80% adoption rate around this time last year. This shows that once again, the adoption rate for the new iOS is trailing behind previous versions of the operating system. These numbers are still astounding compared to other mobile operating systems.