Public Discourse in the Web Does Not Exhibit Group Polarization
  We performed a massive study of the dynamics of group deliberation among several websites containing millions of opinions on topics ranging from books to media. Contrary to the common phenomenon of group polarization observed offline, we measured a strong tendency towards moderate views in the course of time. This phenomenon possibly operates through a self-selection bias whereby previous comments and ratings elicit contrarian views that soften the previous opinions. 
Fang Wu and Bernardo A. Huberman