Online impressions from word of mouth recommendations are about a quarter of the level or paid advertising impressions

"People's influence on each other rivals online advertising. For comparison, for a 12-month period ending September 30 last year, Nielsen Online estimates advertisers created 1.974 trillion online advertising impressions, compared to the 500 billion impressions people make on each other about products and services. So people's online impressions on each other about products and services are about one-fourth of the online advertising impression. And peer impressions are more credible than advertising, since they come from friends. Take those numbers into your next budget meeting!"
Source: Josh Bernoff of Forrester, writing in AdAge, 20th April 2010