According to PQ Media, the major reasons for word of mouth marketing's 37.7% growth in 2007 include a decline in consumer use of traditional media, a consumer shift towards digital alternative media, and the fact that consumers are increasingly less engaged with traditional media.
http://www.womma.org/blog/2007/11/word-of-mouth-marketing-one-billion-dollar-industry-in-2007-expected-to-grow-to-37-billion-by-2011/http://adage.com/article?article_id=122025