Millennial Media is out with its July Mobile Mix report. The mobile ad network releases the report monthly, covering activity on its network. This most notably includes the mobile devices, operating systems and apps where it’s seeing the most ad requests and impressions.

Though this isn’t directly representative of the entire market — including only activity on its network — it’s directionally relevant to see how mobile advertising activity is shifting over time. It’s also a fairly large sample if you consider that Millennial reaches 63 million of the 77 million U.S. mobile Web users (Nielsen).

Here are top takeaways from the report:

— Apple remains the largest smartphone OS in Millennial’s network with 55 percent of impressions, a 1  percent month-over-month drop

— Android beat out RIM for the first time, holding the second-highest share of impressions at 19 percent, an 8 percent month-over-month increase

— RIM’s comes in third with 16 percent of impressions, a 1 percent month-over-month drop

— In terms of ad requests, Apple’s iOS ad requests grew 24 percent month-over-month, and have grown a total of 15 percent since the beginning of the year

— Android is growing fastest in ad requests at 47 percent month-over-month and 690 percent since the beginning of the year.

— RIM ad requests grew 18 percent month-over-month and have increased 66 percent since the beginning of the year.

— In terms of app categories, games took the top spot for the third month in a row with 47 percent of total in app ad revenues on Millennial’s network.

— Social networking came in second and music-related apps are third with 9 percent of app revenue — growing quickly from the sixth position it held in June

Millennial remains the largest independent mobile ad network, as many of its large competitors — like AdMob and Quattro — have been acquired. It has hinted that it not only wants to stay independent, but also will possibly go public next year. I’m meeting with members of the Millennial team tomorrow and will have more to report then.

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Related: WSJ reports on rumors of RIM eyeing Millennial as an acquisition target. This would likely be to match Apple’s capability with iAd (via Quattro acquisition) to monetize its app marketplace and offer revenue incentives to its app developers/publishers. It would also fall in line with our 2010 predictions, including continued M&A among mobile ad networks