Mobile ad network operator Millennial Media is out with its monthly report for May 2011, and this time, the most interesting figure may be the trend related to Windows Phone. According to its data, Microsoft’s Windows Phone operating system (OS) saw a huge increase in ad impressions last month – up 92% month-over-month, says Millennial. However, the OS only has a 1% share of smartphones on its network, trailing far behind smartphone leaders Android (53%) and iOS (27%).

Apple, HTC, Samsung, RIM

On the charts, the Apple iPhone again retained its number one position as the top smartphone for the 20th month in a row. Apple was also the leading manufacturer on the network.

HTC moved up to the number 4 position, and now has almost 11% market share in the top manufacturers list. Samsung is the number 2 OEM and has 6 devices in the top 20 phones list.

Meanwhile, RIM grew 6% month-over-month to attain 17% of the Smartphone OS mix impression share. This growth was attributed to an increase in BlackBerry Curve devices on the network.

Feature Phones Show Increase

In May, Millennial also saw feature phones experience a 6% increase month-over-month, to account for 17% of the Smartphone, Feature Phone and Connected Device impression share. (See top chart).

Smartphone OS Share

In terms of smartphone OS’s, Android maintained its number one position for the 6th month in a row, with 53% of the Smartphone OS mix impression share.

At 43%, Android was also the second largest platform on the Application Platform Mix in May, which ranks platforms by app revenue.  Millennial says it expects Android to start to steal even more share from Apple as developers begin to create more apps for the Android platform in the future.

App Categories

The Music & Entertainment app impressions grew 14% in May, to take over the number 1 spot on the Top 10 Mobile Application Categories ranking. (Games was number 1 last month). This growth is due to the increased use of mobile devices for entertainment, says Millennial.  Music apps still drove most the impressions within this category.

As people continue to plan summer vacations, travel apps also saw a 33% increase, month-over-month.

sarah perez