• by Mark Walsh  @markfwal, October 25, 2011

Android has long since overtaken Apple’s iOS as the largest smartphone platform. But the two top mobile operating systems are still battling as the leader in ad spending within applications on Millennial Media’s mobile ad network. Android was back on top in the third quarter, driving half (49%) of in-app ad revenue compared to 41% for iOS -- the exact reverse of their relative shares in July.

ABI Research on Monday separately reported that Android in the second quarter had, for the first time, surpassed iOS in app downloads -- 44% to 31%. That’s partly because iPhone shipments in the quarter increased only 9% compared to 36% growth for Android device shipments. Android’s installed base now exceeds iOS by a factor of nearly 2.5 to 1, according to ABI.

Android remained the leading operating system on Millennial’s network in the third quarter, accounting for 56% of smartphone ad impressions, compared to 28% for iOS and 13% for BlackBerry. Smartphones overall generated the vast majority of impressions (72%), with connected devices and feature phones accounting for 14% share apiece, according to the network's latest Mobile Mix report. 

When it comes to tablets, the iPad still dominates, responsible for all but 3% of impressions. Its smartphone cousin, the iPhone, was the leading device in its category with 12.5% of impressions. It was followed by the LG Optimus (6.3%), BlackBerry Curve (4.7%), Motorola Droid (4.3%) and HTC Desire (4%).

Android, however, made up 15 of the top 20 phones on the Millennial network. Six of those phones were made by Taiwan-based manufacturer HTC, twice the number from a year ago. In terms of manufacturers overall, Apple was again the top brand with a 23% share of impressions. Samsung had 16%, followed by HTC (15.5%), Research in Motion (11%), and Motorola (10.7%).

Among app categories, gaming remained the most popular, with a 34% share of impressions. That’s up 26% from a year ago. News also broke into the top five app verticals, with 36% quarterly growth. Other leading categories in the third quarter included music and entertainment, social networking and communications.