A couple recent studies by BIGresearch that look at purchase influence factors across all media - traditional and online - shed further light on the importance of the personal advice network. As a pair, they paint an interesting picture: person-to-person product recommendations are the #1 influence factor on buyers (at least for a number of product categories), and they are the preferred way for recommenders to deliver their advice.
One study, conducted for the Retail Advertising and Marketing Association, shows that word-of-mouth is the top influence channel for electronics (44.4%) and apparel purchases (34.3%). In electronics, product reviews came in second (36.8%) and retail advertising inserts came in third (29.2%). In apparel, retail advertising inserts came in second (33.3%) and in-store promotions came in third (30.4%).
The other, reprinted in eMarketer, shows that across all age groups, the primary means through which people communicate product recommendations are the traditional ones - face-to-face, email, and phone - with blogs and communities far behind. What's significant here is that the leaders are primarily point-to-point channels, not mass communications channels, highlighting that people with advice to offer prefer to deliver it directly to the person who needs it. (OK, people could be email blasting their product recommendations to their friends, but I suspect the dominant mode here is more targeted.)