Disagreement persists as to whether social networking sites (SNSs) are used more frequently to facilitate cross-cutting or like-minded discussions. We examine the relationship between the use of SNSs and involvement in discussions with politically similar and dissimilar others among a sample of US Democrats and Republicans. Affective polarization is negatively related to involvement in cross-cutting discussions, suggesting that individuals extend their dislike of the opposing political party to out-party members within their online social networks. Moreover, political discussion with one’s friends on SNSs plays a mediating role in involvement in both cross-cutting and like-minded discussions. Finally, party identification moderates the relationship between SNS use and involvement in cross-cutting discussions, indicating that Republicans participate more frequently than Democrats in cross-cutting exchanges on SNSs. In the light of these findings, we discuss the contribution of SNSs to the ideals of deliberative democracy.