There's a hitwise report on participation in social media (dated November on their list of Reports, but February in the file) that's getting traction because of a presentation at the Web 2.0 conference last week. it seems to confirm these numbers; see http://www.hitwise.com/registration-page/us-cgm-report.php to register for a copy of the report

j. trant archives & museum informatics www.archimuse.com