Word-of-mouth marketing often targets influencers — people who are passionate about a product category, and who are perceived as credible sources. The theory is that reaching more influencers increases the odds that they will spread the message.

The theory's appeal is clear to some. Advertisers are far down on the list of trusted information sources, according to a Bridge Ratings/University of Massachusetts study published in August 2007. Getting just about anybody else besides advertisers to convey the message would seem to be more effective.

The study also revealed that 93% of respondents said they were moved to take some sort of action by WOM influence.

eMarketer estimates that 20% of US adults will be WOM influencers in 2011, up from 17.5% in 2007. Web sites, blogs and other technology are making it easier to spread the word on a favorite topic.

One important attribute of people with influence, in addition to their use of word-of-mouth is their reliance on many kinds of media for information.

Influencers are more likely than the general population to use their local newspaper and cable TV news. Online, they use broadcast news sites and blogs more often, according to Ketchum and the Annenberg Center.

Influencer affinity for online media makes them an especially attractive target for interactive marketers.