Some 24 percent of the 34.3 million US child and teen internet users will visit virtual worlds once a month in '07, according to eMarketer, which expects that 34 percent will do so in 2008, and 53 percent by 2011, writes MarketingCharts.

In absolute numbers, that's 8.2 million kids and teens expected to visit virtual worlds this year, 12.0 million in 2008, 20 million in 2011.

Combining aspects of video gaming, social networking and communicating, virtual worlds have appeal for both genders and are an intriguing opportunity for those marketing to kids and teens, according to eMarketer's latest report, "Kids and Teens Online: Virtual Worlds Open New Universe."

(Also see "Social-Network Advertising to Keep Growing.")

Kids are online in record numbers, with 41.5 percent of kids age 3-11, or 14.9 million, connecting to the internet at least once a month this year, according to eMarketer, which expects those numbers to increase to 16.6 million, or 44.3 percent, by the end of 2011.

The proportion of online teens (age 12-17) is significantly higher: 76.4 percent are estimated to go online at least once a month in 2007, and that proportion will increase to 87.1 percent in 2011, eMarketer forecasts; that's from 19.4 million in 2007 to 21.1 million in 2011.

Children and teens together comprise 18.2 percent of US Internet users, according to eMarketer.

The good news for marketers is that most virtual worlds are capable of offering detailed information about how their users interact with brands and advertising.

"The bad news," said Debra Aho Williamson, senior analyst and author of the report, "is that it is difficult to know what all this virtual interaction really means. What value is there in a person's avatar drinking a Pepsi? Or wearing a shirt bought from a virtual store? What if a person's virtual activities have no bearing on their real-world activities?"