Though the Apple iPhone remains the top device, based on impressions, the iOS is second, once again, to Android. Mobile ad network Millennial Media released its July Mobile Mix Report, and the Android operating system grew 15 percent month-over-month in terms of smartphone impressions with 61 percent. This is the eighth month in a row that Android has been on top.

According to Millennial, iOS held 21 percent of smartphone impressions, followed by RIM at 14 percent. Behind them were Symbian and Windows at 2 percent each. Interestingly enough, however, Windows Phone 7 actually grew significantly this past month with a global month-over-month growth of 71 percent.

Despite Android holding significantly more OS impressions than iOS, the ad spend difference from applications between the two was not as great. While Android was still highest, with 48 percent coming from Android apps, iOS was just behind it with 43 percent (a 5 percent month-over-month growth).

Of all app impressions, games continue to be the highest contributors and have been so for the past year. They continue to grow and actually increased in impressions, month-over-month, by 35 percent to make up 29 percent of total application impressions. Behind games was music and entertainment followed by mobile social networking. Health and fitness apps were also highlighted with a 13 percent month-over-month growth, though they are ranked at #7.

In terms of device breakdown, and following the Apple iPhone, Millennial Media reports that the #2 handset is the Android-powered Samsung Nexus S (Samsung as a manufacturer actually grew 28 percent as a whole), followed by the BlackBerry Curve and the Motorola Droid.

Millennial also added a new set of metrics, reporting on Wi-Fi impressions. In July, the majority — 33 percent — of impressions came from Wi-Fi (multiple carriers) connections. Regarding carriers, however, Verizon held 19 percent followed by Sprint – Nextel at 13 percent.