We present a study of 10-month Twitter discussions on the controversial topic of Edward Snowden. We demonstrate how behavioral differences of opinion groups can distort the presence of opinions on a social media platform. By studying the differences between a numerical minority (anti-Snowden) and a majority (pro-Snowden) group, we found that the minority group engaged in a "shared audiencing" practice with more persistent production of original tweets, focusing increasingly on inter-personal interactions with like-minded others. The majority group engaged in a "gatewatching" practice by disseminating information from the group, and over time shifted further from making original comments to retweeting others'. The findings show consistency with previous social science research on how social environment shapes majority and minority group behaviors. We also highlight that they can be further distorted by the collective use of social media design features such as the "retweet" button, by introducing the concept of "amplification" to measure how a design feature biases the voice of an opinion group. Our work presents a warning to not oversimplify analysis of social media data for inferring social opinions.