The IAB have produced a 15-page document titled 'User Generated Content, Social Media and Advertising - An overview' to help marketeers and agencies understand the landscape and the opportunites.
There is some decent information here and useful if you are new-ish to social media.
One point I found interesting was the Nielson research from last Oct which claims only 34% of consumers trust search engine ads. Another I hadn't heard before was that the average LinkedIn user is 39 years old with an annual income of $139,000.
Thanks to Alan Moore for original post.
Download the report here at IAB's site.