I found this fascinating quote today:
Recently, Forrester introduced a new way to consider influence in Social Media. We identified a group of social media participants we call Mass Influencers. While just 16% of the US population, Mass Influencers are responsible for 80% of the influence impressions and posts about products and services in social channels. blogs.forrester.com, Peer Influence Analysis: What It Is & How Marketers Use It, Apr 2010
You should read the whole article.