  Social media, such as blogs, are often seen as democratic entities that allow
more voices to be heard than the conventional mass or elite media. Some also
feel that social media exhibits a balancing force against the arguably slanted
elite media. A systematic comparison between social and mainstream media is
necessary but challenging due to the scale and dynamic nature of modern
communication. Here we propose empirical measures to quantify the extent and
dynamics of social (blog) and mainstream (news) media bias. We focus on a
particular form of bias---coverage quantity---as applied to stories about the
111th US Congress. We compare observed coverage of Members of Congress against
a null model of unbiased coverage, testing for biases with respect to political
party, popular front runners, regions of the country, and more. Our measures
suggest distinct characteristics in news and blog media. A simple generative
model, in agreement with data, reveals differences in the process of coverage
selection between the two media.
