Last year was the year that Android soared and feature phones went into free-fall, according to Millennial Media’s year-end Mobile Mix Report.

Over the course of the year, Android surpassed iOS, becoming both the fastest growing mobile OS and the leading smartphone platform. Overall Android’s OS market share increased by 57 percent year-on-year, growing from 30 to 47 percent. The total number of Android ad impressions increased by 504 percent between the end of 2010 and 2011.

Android’s gains last year came mostly at the expense of Apple’s iOS, which dropped to second place, seeing its market share decline from 41 percent to 33 percent between 2010 and 2011. RIM and Symbian were also affected by the rise of Android, seeing their market share decline to 16 and one percent respectively. Windows Phone held steady with two percent of market share in both 2010 and 2011.

The other key trend of 2011 was the rapid decline of feature phones. The devices took a big hit, with their share of total impressions declining from 31 percent to 17 percent in just 12 months. While the drop may be dramatic, because Millennial’s data has been averaged across 12 months the decline is also downplayed slightly — according to Millennial’s November Mobile Mix Report, feature phones had actually declined to 14 percent of Millennial’s impression share.

The fall of feature phones was also reflected in Millennial’s device charts. In 2010, Millennial recorded three feature phones in its top 20 most popular devices: The Samsung Freeform (No. 4), the Sanyo Incognito (No. 8) and the Motorola Clutch (No. 14). By the end of 2011 only the Samsung Freeform remained in the No. 16 spot.

The iPhone was the most popular phone overall in both 2010 and 2011, but its share of impressions dropped by seven percent as the market share of Android devices from manufacturers like Samsung and HTC increased.

Other connected devices (tablets, eReaders, iPod Touches, etc) saw only a marginal increase, from 14 to 15 percent of total impressions between 2010 and 2011, but posted a 10 percent increase in the total number of impressions year-on-year. Millennial attributed the gain to the popularity of the iPad despite the fact that more than a dozen tablets were released during the year. The second most popular tablet in 2011 was the Samsung Galaxy Tab, which has just over half of the impression share of Apple’s iPad.

As far as the type of apps that Millennial’s ads were displayed in, games saw their share of impressions increase by 16 percent year-on-year to take the No. 1 spot, likely spurred on by the growth of Android, where developers are far more reliant on advertising to monetize than on iOS. Music and entertainment apps were the second most popular mobile application category by share of impressions and last year’s leader, social networking apps declined to third place. Health and fitness apps tripled their share of impressions year-on-year to rise to the No. 9 spot on the list.