The Economist has a special section this week on social networking.  They include the chart below pointing out that friend recommendations are the most trusted source of product information.  What's interesting here is just how far ahead friend recommendations are from the next closest source; eyeballing the "trust completely" bar, the factor appears to be almost 3X.

So if trust is important in your sales and marketing, you really need to be thinking about how to harness the power of social networks.