Is social media the main cause of political polarization?

Link to the article: http://www.vox.com/policy-and-politics/2017/4/12/15259438/social-media-political-polarization

Based on this research (http://www.nber.org/papers/w23258), this article argues that it is not social media that polarizes the politics. The research studies how political polarization changed between 1996 and 2012. The result was that eight out of the nine measures of political polarization increase more in the old people than in the young people. Since social media is much more popular among the young people than the old people, this seems to suggest that social media is not the main reason that has caused political polarization. As written in the article, "if Facebook is the problem, then how come the problem is worst among those who don’t use Facebook?" The researchers then proposed two hypotheses for the actual main cause of polarization: one is structural issues, like income inequality, and the other is non-digital media. After all, just as Facebook users can live in a bubble, people can also choose to only subscribe to media outlets whose political views they agree with.