Celebrity endorsements have long been the domain of TV and magazine ads. But now, marketers have discovered that stars and other so-called influencers can help move product just as well through their social media channels. Sixty-one percent of marketers surveyed by Izea, a social sponsorship marketplace, said they’ve paid someone to mention their product on social media, up 5 percent over 2012. But marketers who think social sponsorship is a cheap way to move product are in for a surprise. This new breed of pitchman puts a high value on their sponsored videos and blog posts, and expects to get compensated in cash, not freebies.