Employing a national probability survey in 2012, this study tests relationships between social media, social network service (SNS) network heterogeneity, and opinion polarization. The results show that the use of social media is a positive predictor of the level of network heterogeneity on SNSs and that the relationship is mediated by several news‐related activities, such as getting news, news posting, and talking about politics on SNSs. Testing the association between SNS network heterogeneity and polarization, this study considers 3 different dimensions of opinion polarization: partisan, ideological, and issue. The findings indicate that political discussion moderates the relationship between network heterogeneity and the level of partisan and ideological polarizations. The implications of this study are discussed.