U.S. smartphone users spend an average of four hours and 38 minutes per month browsing the Web, driven primarily by social networking and Internet commerce, a market research firm said Wednesday.

The time spent by U.S. smartphone fans is almost twice that of users of the advanced mobile phones in Britain, M:Metrics reported. In the U.S., the mobile Web users spend an average of one hour and 39 minutes a month browsing Craigslist, the longest time on any site among the top 20 domains visited. In the United Kingdom, the biggest draw is Facebook, where people spend an average of one hour and 45 minutes.

In the U.S., Facebook ranked fourth in terms of time spent browsing, after eBay and MySpace. Rounding out the top five was Disney's Go.com. In the U.K., the remaining top sites were mobile operator 3's portal, Sky TV, Microsoft's Live, and the BBC.

Mobile browsing among smartphone users in the U.S. has increased 89% year over year, and page views have risen 127%, M:Metrics said. "Consumption is quickly evolving from brief transactions, such as checking the weather or flight status, to time-intensive interaction with mobile Web sites," M:Metrics analyst Mark Donovan said in a statement.

A primary reason for the discrepancy in usage between the United States and the United Kingdom is the relative popularity of flat-rate data plans in the former, where 10.9% of users have unlimited plans versus only 2.3% in Britain, the researcher said.

Nevertheless, social networking is driving increasing usage in Britain. On the days they visit the sites, users spend an average of 19 minutes on Facebook, compared to 15 minutes on Microsoft Live, 14 minutes on Sky TV, 10 minutes on the 3 portal, and nine minutes watching the BBC, M:Metrics reported.

In the United States, people spend an average of 29 minutes on eBay, 22 minutes on Craigslist, 18 minutes on Go.com, 16 minutes on MySpace, and 14 minutes on Facebook.

The report is based on data derived from users with Windows, Symbian, and Palm handsets. M:Metrics uses on-device metering technologies to capture mobile browsing and messaging activities among a panel of 3,500 smartphone users in the United States and the United Kingdom.