During the second quarter of 2008, a typical U.S. mobile subscriber placed or received 204 phone calls each month.  In comparison, the average mobile customer sent or received 357 text messages per month — a 450% increase over the number of text messages circulated monthly during the same period in 2006.

U.S. teens (ages 13 to 17) had the highest levels of text messaging in Q2 2008, sending and receiving an average of 1,742 text messages per month.  In comparison, teens took part in an average of 231 mobile phone calls per month, during the same time period.

Nielsen tracks billing activity through an opt-in panel of more than 50,000 personally liable, postpaid U.S. mobile lines across the top four carriers, AT&T, Sprint Nextel, T-Mobile, and Verizon Wireless.

Read more about text messaging trends in the U.S. in the November issue of Nielsen’s “Consumer Insight” online newsletter.

Read coverage of Nielsen’s findings in The New York Times, Wired, the St. Louis Post-Dispatch, Cellular-news.com, Gearlog, and on CNET.com.

The report found that mobile Internet penetration is highest in the U.S., where 15.6% of wireless subscribers use the Internet on their mobile devices. The U.K. followed, with a 12.9% mobile Internet penetration rate, while Italy took the third spot with an 11.9% penetration rate.

Those rates indicate that the mobile Internet market is mature enough to attract advertising, but it may take time for advertisers to migrate to the mobile Web. ”I think marketers need to continue to be convinced to take the leap into the mobile internet,” Nic Covey, Director of Insights, Nielsen Mobile, told the Post.

Read the full report.