• by Mark Walsh  @markfwal, April 23, 2009

In its latest monthly metrics report, mobile ad network ad network AdMob reveals that the Android-powered G1 from T-Mobile is now the fourth leading smartphone in the U.S. in terms of mobile ad impressions behind the iPhone, BlackBerry Curve and BlackBerry Pearl. But the 72 million impressions the G1 generated in March were still small compared to the 607 million generated by the iPhone, representing half of U.S. smartphone traffic. 

An estimated 1 to 2 million G1 phones have been sold so far in the U.S. compared to about 15 million iPhones. Continued strong iPhone sales helped buoy the first-quarter earnings of both Apple and exclusive service provider AT&T.

But as more Android-based phones are sold around the world and the Android operating system extends to netbooks and other devices, AdMob expects that gap with the iPhone to close over time.

The report looked specifically at how the traffic growth of Android devices compared to the iPhone's growth at launch. It found that Android traffic in the U.S. grew an average of 47% per month since the Google platform launched in November, while iPhone traffic increased an average of 88% in the first five months after the launch of Apple's App Store.,

Given Apple's head start with millions of devices already sold and a big marketing push, the iPhone's faster growth is surprising. But applications have had a significant impact on the growth of both mobile competitors, with more than half the total number of ad impressions coming from applications.

"And these apps are not just games, but movie apps, sports apps, news apps, and more that are likely to replace some browser activities," according to an AdMob statement. "This means that yes, application stores on other platforms can be successful."

The AdMob report is based on ad requests the company receives from its network of more than 6,000 mobile Web sites and 1,000 iPhone and Android applications.