1. A method implemented by data processing apparatus, the method comprising: determining, by the data processing apparatus, and for each of one or more topics associated with the user, an expertise score indicating a level of expertise the user has with the respective topic, a responsiveness score indicating responsiveness of the user to previous questions routed to the user about the respective topic, and a quality score indicating a quality of answers, by the user, to the previous questions about the respective topic; obtaining, by the data processing apparatus, one or more additional topics, not explicitly associated with the user; determining, by the data processing apparatus, and for each of one or more additional topics, an expertise score indicating a level of expertise the user has with the respective additional topic; associating, by the data processing apparatus, and for each of the one or more topics and the one or more additional topics, the expertise score, the responsiveness score, and the quality score with the user; and selectively routing, b the data one or more queries to the user in response to the user being identified as a candidate answerer from a plurality of users based on one or more of the expertise score, the responsiveness score and the quality score.