Word of mouth, sometimes referred to as buzz marketing or viral marketing, was the fastest-growing slice of the $254 billion marketing industry last year, and is expected to account for more than $1 billion of ad spending in 2007, according to a report by PQ Media of Stamford, Conn., an alternative media researcher. That number is forecast to reach $3.7 billion by 2011, fueled in part by the eruption of blogs and the increasing popularity of social networking sites such as Facebook.

Word of mouth, paradoxically, is one of the oldest forms of advertising. As long as there have been brands, people have talked about them, both positively and negatively. Yet it is only in the past few years that a diverse range of marketers have begun to make it a regular staple of their brand strategies. For one thing, it is relatively cheap. More important, it is trusted.

“It totally outweighs all the other forms of advertising and marketing direct in terms of trust,” said Leo Kivijarv, vice president of research for PQ Media. A recent survey by A.C. Nielsen found that 78 percent of respondents viewed recommendations from other consumers as trustworthy. That compares with 63 percent for newspaper ads, the second most-trusted medium, and well above the 18 percent for text ads on mobile phones.

Trust, however, can be a double-edged sword. Nothing can kill a brand faster than bad word of mouth, precisely because feedback from other consumers is viewed more credibly than a conventional piece of advertising.