A study released today shows 15.6 percent of mobile Internet subscribers use the mobile Web -- the highest penetration of any country. With 40 million users, the firm believes the mobile Web has reached 'critical mass' for mobile ads.

Mobile Web usage expanded from 22.6 million users to 40 million here in the US since 2006. There is room for much growth: as many as 95 million may be paying for mobile Web services but not using them.

Of the 16 countries tracked in the survey, the US led in adoption, followed by the UK with 12.9%, and Italy 11.9%.

The firm also found that Yahoo Mail's mobile site was the most popular destination. An estimated 14.2 million used this service during May, with Google search coming in second at 9.1 million. Rounding out the top five were The Weather Channel (purchased earlier this week by NBC Universal) with 8.6 million, MSN Hotmail with 7.9 million, and Gmail with 7.5 million.

Mobile Web surfers visit an average of a little over 6 different Web sites every month, a fraction of the more than 100 unique sites desktop surfers visit.

While the iPhone is certainly playing a big part in advancing mobile use of the Internet, it is the Motorola RAZR which tops usage in the US. Across the Pacific in Asia, consumers seem to prefer Nokia's line of handsets.

Even with a small penetration in the market, the iPhone is driving usage. 82% of users were accessing the Internet, a rate that is five times higher than its competitors.

In any case, mobile advertising appears to be the next frontier. A quarter of mobile Web users reported they viewed a mobile ad, fueling the firm to say that the industry had reached a 'critical mass.'

"A confluence of factors will only further ignite this market this year and into 2009," director of insights Nic Covey said. "We believe the audience, inventory and planning tools are in place to make mobile Internet marketing an important part of the media consideration set today."