Think of it as an ISN within an ESN. According to e-Marketer's December 2007 Social Network Marketing: Ad Spending and Usage report, social networking is an activity that 37 percent of U.S. adult Internet users and 70 percent of online teens engage in every month, and the numbers continue to grow. eMarketer projects that by 2011, one-half of online adults and 84 percent of online teens in the U.S.