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Even Scoble couldn't save Seagate. Almost a year after the hard-drive maker renewed a sponsorship deal with the prolific blogger, its stock is down 35 percent. Archrival Western Digital, meanwhile, is up 40 percent. So much for the profession of "influencer marketing," a field which has exploded since the 2000 publication of Malcolm Gladwell's The Tipping Point and the subsequent work The Influentials. These books, translated into action by marketers, have prompted companies from AT&T to Yahoo to hire executives expressly to suck up to bloggers. Seagate's Scoble sponsorship is the purest expression of this trend. And the best illustration of why it doesn't work.The theory it's based on is nonsense. It is true that ideas spread virally through the population. But it turns out that there's not a single set of influencers who are reliable Typhoid Marys. Duncan Watts, a former Columbia University researcher who now works for Yahoo, found in a study that the emergence of contagious ideas is random. Repeated experiments found that anyone can start a trend, and it's impossible to predict who those people will be. Watts's research is not 100 percent conclusive; his models might not translate perfectly to the real world. So let's go there! In April, a study by Canadian research firm Pollara found that word of mouth works — nearly 80 percent said they'd buy products recommended by a friend or family member. But word of mouse? Only 23 percent said they'd buy something touted by a blogger. "This shows that popularity doesn't always equate to credibility," Pollara executive Robert Hutton told MediaPost. "Marketers might have to reconsider who the real influencers are out there." In backing Scoble, Seagate hoped to buy cheap buzz. It's a convenient fantasy for marketers: Find the one magic guy to woo, then watch him chat up your company to Wall Street traders! Seagate would have been better off sending big hard drives to a dozen bloggers. Or a hundred. Or, for that matter, a random assortment of people, whether or not they have a habit of typing out the contents of their brain every 3 seconds. Anyone — literally — would have been a better choice than Scoble.