Mobile ad network Millennial Media has released its monthly Mobile Mix report for March 2011, providing an insight to the mobile market as a whole. Notably Android is still on top, but many of its competitors have seen a lot of month-over-month growth.

For the 4^th month in a row, Android remains the leading OS with a 48 percent impressions share on Millennial’s ad network. And, according to executive director and senior research analyst at UBS Investment Bank Brian Pitz, Android will most likely maintain this position – according to him, Google activates 350,000 Android devices every day. If accurate, this figure represents tremendous growth for the internet giant’s mobile OS;  the number, after all, was 200,000 two months ago according to Google chairman Eric Schmidt.

Heading over to the iOS front, Apple’s iPhone grew 17 percent month-over-month in March, while the number of impressions from all iOS devices grew 11 percent in the same period. Despite of Android’s dominant ad impression share, Apple still accounts for the most app revenue, with a 47 percent market share in March compared to Android’s 36 percent.

Fourteen of the top 20 devices on Millennia’s network are running Android, but it’s important to remember 2 are made by RIM. The BlackBerry Curve reclaimed the number 2 position in the ad network’s top 20 list, and the BlackBerry Torch grew 23 percent month-over-month while Blackberry OS grew 29 percent in the same period. Research in Motion has seen a great deal of global growth beyond the Millennial network as well, but Mike Lazaridis, one of the chief executives of the company, is nevertheless upset about the lack of appreciation people have his company’s growth efforts.

Last but not least, the connected devices category – meaning devices such as the non-phones such as the iPod and tablets (including RIM’s Playbook set to launch 5 days from now), increased 21 percent month-over-month. These devices now make up 17 percent of the total impression rates on Millennial’s ad network.