Google Inc.’s Android has overtaken Apple Inc.’s iOS in the wildly growing arena of app downloads. In the second quarter of 2011, 44% of all apps downloaded from app marketplaces across the web were for Android devices while 31% were for Apple devices, according to new data from ABI Research. The remaining apps were for various other mobile operating systems, including BlackBerry and Windows Phone 7.

“Android’s open source strategy is the main factor for its success,” says Lim Shiyang, research associate at ABI Research. “Being a free platform has expanded the Android device installed base, which in turn has driven growth in the number of third-party multiplatform and mobile operator app stores. These conditions alone explain why Android is the new leader in the mobile application market.”

Recent quarterly smartphone shipment growth figures also explain Android’s ascent to the top. The growth rate for iPhone shipments dropped to 9% from the previous quarter’s 15%, ABI says. Android smartphone shipments increased 36% in Q2 2011 compared with 20% in Q1 2011. The number of Android users now exceeds the number of iOS users by a factor of 2.4-to-1 worldwide; by 2016 this factor will grow to 3-to-1, ABI projects.

“Despite leading in total mobile application downloads, Android’s app downloads per user still lag behind Apple’s by 2-to-1,” adds Dan Shey, practice director, mobile services. “Apple’s superior monetization policies attracted good developers within its ranks, thus creating a better catalog of apps and customer experience.”

Global app downloads for year-end 2011 are expected to skyrocket to 29 billion compared with nine billion in 2010, ABI predicts. This massive increase is largely due to the proliferation of smartphones around the world, ABI says.