What's social media good for? Marketers see it as a new way to engage with consumers. Economist turned advertising executive Jason Harper sees an additional function: as a real-time laboratory for measuring how multimillion-dollar ad campaigns are succeeding or failing to drive product sales.

Armed with a master's degree in applied economics, Harper was hired three years ago, at age 30, by the Detroit office of Organic, a leading digital ad agency, to crunch campaign data for car companies--with the goal of seeing whether digital marketing efforts were helping move vehicles off lots. Assigned to Chrysler's Jeep and Dodge Ram truck accounts, Harper had to figure out how to gauge whether TV commercials were increasing website visits, Twitter conversation, and activity on Facebook brand pages. He also had to calculate whether that online activity was leading to an increase in test drives at dealerships. "The biggest question with social media is 'What's the value?'" he says.