The share of adult Internet users who have a profile on an online social network site has more than quadrupled since 2005, from 8% to 35%, according to a November–December 2008 Pew Internet & American Life Project survey.

Pew said adults still make up the bulk of social networking site users. But younger online adults were much more likely than their older counterparts to use social networks, with three-quarters of those ages 18 to 24 using the sites, compared with just 7% of Internet users ages 65 and older.

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While Pew measured adult Internet users who had created a profile on a social networking site, the potential audience for social networking is much larger if occasional visitors are included. In September 2008, 70.2% of US Internet users ages 15 and older visited an online social network, according to comScore World Metrix.

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“The ongoing headache for marketers is that social networking is such a powerful consumer activity, but incredibly challenging as a marketing medium,” said Debra Aho Williamson, senior analyst at eMarketer.