Consumers in the U.S. and China own the most iOS and Android devices in the world — more than 290 million of them between the two countries — but China’s skyrocketing adoption rates could see it overtake the U.S. as the country with the most devices by the end of the year, says Flurry.

The company put together its latest report by logging how many devices in each country it detected running apps using its mobile analytics services. Altogether, Flurry tracks more than 200,000 apps running on more than 640 million devices, which Flurry claims allow it to reliably detect more than 90 percent of all the iOS and Android devices currently active in the world.

Year-over-year, more than 30 million new iOS and Android devices were activated in the U.S., but in the same time frame Chinese consumers activated an incredible 100 million new devices. If China keeps activating iOS and Android devices at the same pace, Flurry predicts it active install base will overtake the U.S. as early as the upcoming 2012 holiday season. As of July 2012, Flurry tracked 165 million active iOS and Android devices in the U.S. and 128 million in China. The U.S.’ share of total iOS and Android worldwide app sessions also declined, falling from 52 percent of all sessions in July 2011 to 36 percent in July 2012.

China’s seemingly insatiable taste for iOS and Android devices also makes it the fastest growing market in the world when compared to other fast-growing markets in South America and Eastern Europe. Between July 2011 and July 2012, the number of active iOS and Android devices in China increased by 401 percent. The next closest country was Chile, which registered 279 percent growth iOS and Android devices year-over-year.

Despite its explosive growth, the market for iOS and Android devices is also less saturated in China than it is in the United States. Flurry compared the number of active devices it tracked and compared that figure to each country’s adult population in order to determine which countries had the highest smartphone penetration. By that measure Flurry found that the U.S. has the fifth highest Android and iOS penetration in the world, with enough active devices to 78 percent of its adult population their own device. The world’s most saturated market was Singapore, which has enough active iOS and Android devices to give 92 percent of its adult population their own.

Overall while Flurry’s report doesn’t tell mobile app developers much that they didn’t already know, it still provides some interesting reference points for those who wish to know exactly how much potential is left in the rapidly growing, yet unsaturated Chinese market. While monetizing apps in the country is still a challenge, Flurry’s report shows that even if 0.05 percent of a company’s Chinese users ever monetize, the sheer size of the market alone could make it far more lucrative than many other countries.