updated 10:07 am EST, Tue December 31, 2013
4 percent still using iOS 5 or earlier
Among iOS users, iOS 7 adoption rose 2 points to 78 percent between December 15th and the 30th, according to official Apple data for developers. Correspondingly, iOS 6 usage dropped two points to 18 percent. A consistent 4 percent are still using iOS 5 or earlier, despite many apps now requiring at least iOS 6. They may however be running older hardware not compatible with iOS 6 or 7.
Apple's data is based on App Store usage. Tracking only began on December 1st, at which point 74 percent of people were already on iOS 7, and 22 percent were on iOS 6. iOS 7 was first released in late September, and has been pre-installed on every iOS device shipped since.