Millennial Media, the largest independent ad network is releasing a yearly report on performance of smartphone operating systems on its network in 2011. According to the report, Android continued its growth as a platform with ad impressions growing 504 percent year-over-year on Millennial’s network.

To give you some background, last year, Android had a 47% share of device OS impressions and iOS had 33%. Compare this to the previous year, 2010, when iOS was on top with a 41% share and Android only 30%. iPhone impression share declined 7% year-over-year but held its position as top device when ranked among 20 mobile phones and had 15% share of impressions. According to Millennial, no other device has been able to achieve the level of market share as the iPhone.

The number one tablet on Millennial’s platform since 2010 has been Apple’s iPad. Galaxy Tab from Samsung is second with only half the share of impressions. For the past two years, Smartphones led in the category of Connected Devices, increasing 68%. Apple was top manufacturer on Millennial’s platform last year and the iPhone was top individual mobile phone. As a device manufacturer, Apple’s share of impressions was fairly flat, but Apple iPad impressions had substantial growth. The number two device manufacturer was Samsung with a flat share of impressions. Android manufacturers enjoyed the largest gain with HTC growing 79% year-over-year and came in as number three manufacturer on Millennial’s platform.