More than one-half of the US population surveyed uses social networking sites, according to IDC, but the ad dollars have not followed. The research company found more than 75% of social network site users logged in at least once a week and 57% did so daily. IDC also said more than 61% of those users spent more than 30 minutes per session on social network sites, and 38% remained parked for 1 hour or more.

Good news for marketers, right? Not necessarily. Only 57% of social network site users said they clicked on an ad in the past year, compared with 79% of all Internet consumers.

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