eMarketer estimates that 15.5 million U.S. teen internet users (ages 12-17), or 75%, will use social networking websites in 2009. By 2013, the estimate grows to 17.9 million, or 79% of online teens.

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Teens use social networking sites primarily for staying in touch, making plans, finding new friends and flirting.

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Still, the social networking experience is not always positive. Teens understand that information posted to social networking sites can come back to haunt them. Over 60% of those surveyed acknowledged that the things friends wrote in their profiles could harm their careers. In addition, 48% said they could be embarrassed by what they themselves wrote, and 38% said they regretted some of the items that had appeared on their pages already.

Source: eMarketer
http://www.emarketer.com/Article.aspx?R=1007041