A growing proportion of citizens rely on social media to gather political information and
to engage in political discussions within their personal networks. Existing studies argue
that social media create “echo-chambers,” where individuals are primarily exposed to likeminded views. However, this literature has ignored that social media platforms facilitate
exposure to messages from those with whom individuals have weak ties, which are more
likely to provide novel information to which individuals would not be exposed otherwise
through offline interactions. Because weak ties tend to be with people who are more politically heterogeneous than citizens’ immediate personal networks, this exposure reduces
political extremism. To test this hypothesis, I develop a new method to estimate dynamic
ideal points for social media users. I apply this method to measure the ideological positions of millions of individuals in Germany, Spain, and the United States over time, as well
as the ideological composition of their personal networks. Results from this panel design
show that most social media users are embedded in ideologically diverse networks, and
that exposure to political diversity has a positive effect on political moderation. This result
is robust to the inclusion of covariates measuring offline political behavior, obtained by
matching Twitter user profiles with publicly available voter files in several U.S. states. I
also provide evidence from survey data in these three countries that bolsters these findings. Contrary to conventional wisdom, my analysis provides evidence that social media
usage reduces mass political polarization.