This paper examines tourists' knowledge sharing behavior in social media. Based on social capital theory, we aim to examine the effects of three dimensions of social capital—structural (social interaction ties), cognitive (shared vision) and relational (trust)—for two different types of social media: Facebook and TripAdvisor. We propose a structural model that connects an antecedent (homophily) and a consequence (knowledge sharing through posting) of these main dimensions of social capital. An online survey is conducted with 1200 Spanish consumers. Based on the full sample, our structural equation modeling supports most of the hypothesized paths, while trust does not affect either social interaction ties or knowledge sharing. Examining the difference between Facebook and TripAdvisor, our multigroup analysis finds that neither trust nor shared vision drives knowledge sharing on TripAdvisor, while shared vision affects knowledge sharing in Facebook. In both media, social interaction ties play an important role in motivating users to post comments. The effects of homophily on social interaction ties and on shared vision were statistically greater in TripAdvisor than in Facebook. Homophily did not affect trust in either media. In closing, both theoretical and managerial implications are discussed, important limitations are recognized and future research directions are suggested. Copyright © 2016 John Wiley & Sons, Ltd.