Interesting article in eMarketer called "Word of Mouth Works Worldwide" According to an April 2007 Nielsen study of internet users in 47 markets around the world, 'recommendations from consumers' is the number one type of trusted 'advertising' (78%). Not too surprising. I think we all agree that we'd probably trust a consumer opinion over an advertisement.
Online banner ads came in at 26% and search engine ads came in at 34%. This is a good reality check for small business owners who live and die by advertising.
The great thing about social media marketing is that if you focus on delivering a great product, the customer will do the work for you. Perhaps you should spend more time on improving your products and services and less on banner ads.