A new report from Millennial Media on Tuesday shows that Apple is doing very well in mobile ad revenue — in fact, it’s leading all manufacturers.

[ad#Google Adsense 300×250 in story]Millennial “reports key advertising trends with a focus on mobile manufacturers, devices, operating systems, connected devices and more.” So, to be clear, we are looking only at advertising market share in this report.

When you look at the study’s findings for Application Platform Mix, Apple has 45 percent of the market. Android came in second with 43 percent of the market. Sounds pretty close.

Apple having a lead is good, but there’s one thing that this study doesn’t take into account that we should when looking at the numbers. More people buy apps for iOS than download them for free.

Since Apple doesn’t allow developers to put ads into paid apps, the data looks pretty close. As an overall moneymaker for developers, I’d bet the gap between iOS and Android is much wider.

A report in May showed that of the paid applications in the Google Android Market, downloads are significantly lower than they are for free applications. The percentage of all free applications that have been downloaded less than 100 times is 24.8 percent, and the percentage of paid applications that have been downloaded less than 100 times worldwide is 79.3 percent.