According to the latest report from M:Metrics, social networking and Internet commerce are compelling U.S. smartphone users to spend an average of 4 hours and 38 minutes per month browsing the mobile Web. In Britian, users spend 2.5 hours per month on the mobile Web.

According to the latest data from the measurement firm’s metered smartphone panel, active mobile Web users in the United States spent more time browsing Craigslist on their smartphones, an average of 1 hour and 39 minutes in the month, than any other site among the top 20 domains visited. The top five most visited mobile sites in the United States are: Craigslist, eBay, MySpace, Facebook and Disney’s Go.com.

In the U.K., Facebook is the top mobile site, with visitors spending 1 hour and 45 minutes on the site per month, on average. Top sites after Facebook in the U.K. are: mobile operator 3’s portal, Sky TV, Microsoft’s Live and BBC.

“People are becoming increasingly engaged in the mobile medium,” said Mark Donovan, senior analyst with M:Metrics, in a statement. “Among smartphone users in the United States, mobile browsing has increased 89% year over year, and page views have increased 127%. Consumption is quickly evolving from brief transactions, such as checking the weather or flight status, to time-intensive interaction with mobile Websites – even without an iPhone.”