Subscribers number more than 100, 000, and Beauchamp brags of a 20% monthly rate of growth in the past year, most garnered strictly by word-of-mouth advertising.

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The Gruets expend much effort getting their wines into Santa Fe restaurants, figuring on word-of-mouth tourist advertising.

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The best--and cheapest--advertising, however, is word of mouth.

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According to Comscore, it's the most visited fitness site with 7 million users (162 million page views in January), but Spark has taken a low-key, word-of-mouth approach compared to its big bucks advertising competitors like Weight Watchers.

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The book's success is attributed to bloggers and the sharing of book reviews and recommendations on social media, as well as old-fashioned word of mouth advertising in physical coffee shops, and most probably at the school gates.

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Word-of-mouth remains the most persuasive form of advertising.

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While these word-of-mouth strategies are still minor in terms of advertising spend in comparison with traditional TV and print campaigns, the level of industry interest is disproportionately high, says Ms McEleny.

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This means trying to spread the message by word of mouth still considered the most-powerful form of advertising.

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They have to rely on expensive advertising campaigns and hope for success through word-of-mouth.

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Also, these chains tend to rely, as In-N-Out always has, on word of mouth as their primary form of advertising.

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