Found this article on BusinessWeek.com discussing the major pros and cons with word of mouth advertising. As it states in the article, word of mouth is powerful because consumers trust their friends and their opinions. The only downfall with it is that it doesn’t advertise on multiple channels. However, I think word of mouth advertising is powerful and should be used as often as possible.
http://www.businessweek.com/debateroom/archives/2011/12/word_of_mouth_is_the_best_ad.html
Tulane Social Media Marketing Blog 
I agree. Word of mouth is very powerful, but of course it is more difficult to manage than other forms of advertising.
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