We finished a working paper summarizing findings of two field experiments and a natural experiment on social influence and processes of opinion polarization on the Internet.  You can find the working paper here.  

Title: Micro Influence and Macro Dynamics of Opinions 

Abstract: There is ongoing debate about the effects of social influence on the micro level and resulting opinion polarization on the macro level. We propose a general model that captures prominent, competing micro-level theories of social influence. Conducting a lab-in-the-field experiment, we observe that individual opinions shift linearly towards the mean of the distribution of other opinions. With this finding, we predict the macro-level opinion dynamics resulting from social influence. We test our predictions using data from a second lab-in-the-field experiment and find that social influence reduces opinion polarization. We corroborate these findings with additional field data.