China will supplant the U.S. as the country representing the largest base of iOS and Android users worldwide by the first quarter of 2013, according to a new report by app advertising and analytics firm Flurry. As of October, the U.S. was home to 181 million iOS and Android devices, with China at 167 million.

But that lead won’t last long. The 55 million active iOS and Android devices added in the U.S. in the last year pales in comparison to the 125 million added in China. The latter is the fastest-growing iOS and Android market, up 293% from a year ago, based on Flurry’s tracking of more than 250,000 apps on 750 million devices globally.

Behind China are countries like Vietnam, Colombia, Chile and Ukraine. The U.S. isn’t even in the top 10 as a more mature, saturated market. That's why Apple wants to make inroads into huge emerging markets like China and India with a cheaper iPhone.

The growing adoption of iOS and Android devices outside the U.S. is also reflected in app usage. A year ago, the U.S. accounted for nearly half (48%) of all app sessions globally. In total, 71% of sessions now take place outside the U.S.