Apple has announced that iOS 8 adoption has reached 64% as measured by the App Store on December 22nd. That's an increase of just 1% from December 8 when Apple last measured the number.
Adoption of iOS 8 was initially slower than iOS 7; however, it has been increasing steadily. Holiday sales of the iPhone 6 and iPhone 6 Plus are also likely to increase usage of the new operating system. It will be interesting to see how Christmas Day activations of new devices impact iOS 8's adoption rate.