  Online social networks have emerged as a significant platform for political
discourse. In this paper we investigate what affects the level of participation
of users in the political discussion. Specifically, are users more likely to be
active when they are surrounded by like-minded individuals, or, alternatively,
when their environment is heterogeneous, and so their messages might be carried
to people with differing views. To answer this question, we analyzed the
activity of about 200K Twitter users who expressed explicit support for one of
the candidates of the 2012 US presidential election. We quantified the level of
political activity (PA) of users by the fraction of political tweets in their
posts, and analyzed the relationship between PA and measures of the users'
political environment. These measures were designed to assess the
likemindedness, e.g., the fraction of users with similar political views, of
their virtual and geographic environments. Our results showed that high PA is
usually obtained by users in politically balanced virtual environment. This is
in line with the disagreement theory of political science that states that a
user's PA is invigorated by the disagreement with their peers. Our results also
show that users surrounded by politically like-minded virtual peers tend to
have low PA. This observation contradicts the echo chamber amplification theory
that states that a person tends to be more politically active when surrounded
by like-minded people. Finally, we observe that the likemindedness of the
geographical environment does not affect PA. We thus conclude that PA of users
is independent of the likemindedness of their geographical environment and is
correlated with likemindedness of their virtual environment. The exact form of
correlation manifests the phenomenon of disagreement and, in a majority of
settings, contradicts the echo chamber amplification theory.
