Mobile Internet marketing has reached a critical mass in the United States, with 15.6 percent of the mobile users actively surfing online, according to a new study conducted by Nielsen Mobile, Media Post reported Wednesday.

The main driver to mobile Internet adoption is the growing popularity of unlimited data packages, which about 14 percent of U.S. mobile users have subscribed for.

Moreover, according to the Nielsen report titled "Critical Mass: The Worldwide State of the Mobile Web," half of mobile data users said they prefer the unlimited option, Media Post reported.

As of May 2008, the top U.S. mobile sites with most unique visitors were Yahoo Mail with 14.2 million, Google Search with 9.1 million, and the Weather Channel with 8.6 million.

Meanwhile, 26 percent of mobile online users view ads, while mobile data users on average are less receptive to advertising than all mobile users, according to Media Post.

In terms of mobile handsets, Motorola's Razr phones are the most popular among U.S. mobile online users, while Nokia is top in Europe and Asia.