The Millennial Media ad network ( reports that that for the first time in the company’s history Android surpassed iOS as the largest smartphone operating system on the Millennial network in terms of impression share.

Android OS impression share saw an 8% increase month-over-month and a 46% impression share on the network in December, compared to iOS’ 32% share (down by 6% from November). RIM followed with a 16% impression share for December, down by 3% from November.

Android ad requests grew 141% from quarter one to quarter three. Since January 2010, Android has grown 3130% over the year. Apple requests grew 12% in Q3 to Q4 2010, and since January, Apple increased by 14%. Apple saw more growth with its tablet device, as iPad requests growing by 280% Q3 to Q4 2010.

This month, Millennial broke down the revenue generated by apps in Q4, and interestingly Android had a 55% share as opposed to 39% for Apple, notes “TechCrunch” ( Android apps had a 13% increase in ad revenue quarter-over-quarter.

Though Apple may have lost the top spot in terms of impression share, Millennial says Apple continued its reign as the top manufacturer on the Millennial network (as it has been for the last 15 months), representing 21% of the network’s impression share by manufacturer in December.

In terms of actual devices, the iPhone and iPod touch made up the two of the top three individual mobile devices. Millennial’s ads reach 63 million of a total of 77 million mobile web users in the U.S. — or 81 percent of the U.S. mobile web.