IAB--Social Media The Interactive Advertising Bureau reports on internet advertising revenue, social media, etc. Many useful links.
CMO's Guide to: the Social Landscape Great chart listing the good and bad points of Facebook, Twitter, digg, Flickr, etc. Covers digital trends.
Pew Research Center Has statistics on internet use and trends and attitudes towards social media.
Social Media Marketing Basic social media tips for small businesses. Some statistics provided.
Socialnomics The case studies, graphics, and statistics sections are particularly useful.