Despite a long history, word of mouth as a marketing discipline is only just coming into its own, and the data indicate its best years are yet to come. eMarketer estimates that about half of all online marketers are engaging in some form of word of mouth or viral campaign, and that number will grow.

"While online advertising and all of its many subcategories such as search have dozens of metrics defining their size, shape and growth rates, no one has locked down defining numbers for word-of-mouth marketing," says Geoff Ramsey, eMarketer CEO and author of the Word of Mouth Marketing report. "It has been the 'hidden statistic.'"

Based on aggregated data from dozens of sources, as well as interviews with various word of mouth marketing experts, for the first time, eMarketer — in partnership with the Word of Mouth Marketing Association (WOMMA), uncovers the numbers behind the rumors, the facts behind the myths.

"Word of mouth has always been there, and it's always been the thing that gets us to buy," says Andy Sernovitz, CEO of WOMMA. "What's exciting is that marketers can finally do something to harness this power—and measure the results. Word of Mouth has moved from anecdotal to actionable."

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