Its official that Jelly Bean has been running on 50% Android devices and took place of Android 2.3 Gingerbread. Earlier for some last few years, Android 2.3 Gingerbread was at the same position as that of Jelly Bean today.
Tablets running on Jelly Bean constitute the most of the share also Android 4.3 has shown on the chart for the first time. In real 48.6% of all Android devices are packed with Jelly Bean. Gingerbread still stands on 28% of active devices, which could mean some 280 million if we use Google's recent announcement of 1 billion activated Androids as an indicator.
The statistics are generated by Google using data collected during a 7-day period ending on October 2, 2013. So, if a user has not visited the Android Google Play Store in a month, the device would not be recorded in Google's reports.
Source: Google via Go Android