Users of a social networking system post questions for other users to answer. Questions are automatically tagged based on keywords extracted from text within the posted questions as well as user-selected tags. Users also browse questions asked by other users on the social networking system using an interface that displays questions by topics and sub-topics. Answers may be voted on and sorted by social information related to the browsing user. Affinities for tags are recorded based on users' interactions with the question and answer service. Affinities for tags may also be used to target questions to other users and sort answers.