Mobile Web surfers in the U.S. spend more time on classified-ad site Craigslist than on any other Web site, and they spent nearly twice as much time browsing as their British counterparts in March.

Those are among the findings from a study by mobile research company M:Metrics in which client software installed on participants' smartphones gleaned information about user activity. It found U.S. owners of smartphones -- not even counting iPhones or BlackBerry devices -- spent nearly 4 hours, 38 minutes using their browsers in March. U.K. subscribers spent just under 2 hours, 25 minutes, on average. The survey tracked 3,500 users of Symbian, Palm and Microsoft Windows Mobile smartphones in the U.S. and U.K.

The devotion to mobile browsing in the U.S. has a lot to do with the wider use of so-called "unlimited" data plans in the country, M:Metrics said. In the U.S., 10.9 percent of smartphone users have data plans that don't charge them per bit or per minute for browsing, the researchers said. In the U.K., only 2.3 percent have such plans. Mobile operators in Europe have been slower to adopt all-you-can-eat pricing, said M:Metrics analyst Mark Donovan.

U.S. users spent about 1 hour, 39 minutes per month on Craigslist, M:Metrics said. The site has two big factors in its favor as a mobile destination, Donovan said. It's text-based, so it's easier to load with a weak signal, and it emphasizes local information. Donovan believes mobile users are visiting local listings for events such as yard sales along with the site's popular personal ads.

E-commerce giant eBay was the second most popular destination, with smartphone users spending about 1 hour, 26 minutes per month there, closely followed by social networking sites Myspace and Facebook. The Walt Disney Co.'s Go.com entertainment portal was fifth, with 1 hour, 7 minutes.

Although average time spent per month was higher for Craigslist, individual visits to eBay lasted longer. On the days they visited eBay, users spent an average of 29 minutes there, versus 22 minutes on Craigslist, M:Metrics said.

Facebook led in browser time in the U.K., with an average of nearly 1 hour, 45 minutes per month, followed by the portal of Three, a popular 3G (third generation telephony) operator. Media company British Sky Broadcasting's site was third, followed by Microsoft's Live.com search site and the BBC.

The study covered all Internet visits through dedicated browsers and mobile operator portals, but not through the dedicated mobile client applications that Facebook and others have introduced, Donovan said. M:Metrics hasn't yet brought iPhone or BlackBerry users into the monthly survey. Although the iPhone is the single device used most often for mobile browsing (most mobile OSes are represented by many different devices), Donovan believes the use patterns would look similar if the iPhone and BlackBerry devices were included.