The research firm estimates that the number of children aged 3 to 11 on social networks will grow 61% in the next five years, adults 18 and over, 49.5%, and teens, 20.1%. The overall U.S. social networking population will grow 11% to 88.1 million in 2009.

MySpace says it already has 130 million users worldwide, including 76 million in the U.S. Facebook says it has more 175 million active users, 70% of which are outside the U.S. Facebook has been growing at a torrid pace lately, adding new users at a rate of about 600,000 a day.

Social networking trends highlighted by eMarketer for this year include the expanding user demographics, rising engagement, the impact of mobile social networking and Twitter, and whether the rapid growth of Facebook in the second half of 2008 will continue.

The report also encourages marketers to tap into social networks to engage users as they share product recommendations and buying advice. "This organic flow of information is difficult to track and capture, but companies should find the tools to understand the impact of social network communication on the purchase process," wrote Debra Aho Williamson, a senior analyst at eMarketer who authored the report.