Increasing numbers of citizens rely on social media to gather both political and non-political information. This fact raises questions about belief formation and belief updating in the social media setting. Using Facebook data on users’ behaviour in Poland in 2017, I test the hypothesis that individuals tend to like content that confirms their beliefs. I measure the political preferences of nearly 1.4 million users who were active on the main political and news media pages and classify them as being supporters of certain political organisations or as being politically unaffiliated. Based on the principles of analytical sociology, I construct a theoretical model that may explain the results. According to the model, users tend to like posts from only one source of information. There are also statistically significant differences in the news media preferences of supporters of different political organisations. They are prone to like posts published by sources that accord with their views. The model also correctly predicts that politically unaffiliated users choose media outlets that are considered unbiased or less biased. The results support the hypothesis that users of social media prefer exclusive or near-exclusive sources of information.