In one embodiment, a method includes, in an online social network, accessing one or more first content objects associated with a user in the online social network and a second content object, determining topics and corresponding first weights of the topics for the first content objects using a topic extraction algorithm, where each first weight indicates a strength of an association between the corresponding topic and the first content object(s), determining one or more second weights of the topics for the second content object, where each second weight indicates a strength of an association between the corresponding topic and the second content object, and calculating a similarity score for the second content object based on a comparison of, for each topic, the first weight to the second weight, where the similarity score is to be used in a determination regarding presentation of the second content object to the user.