1. A method performed by data processing apparatus, the method comprising: receiving a query from an asker; before routing the query to at least one candidate answerer: calculating for each of a plurality of users a respective first probability that the user will successfully answer a query by an asker about one or more topics, wherein the first probability is based at least partly on topics in which the user has expertise, and, for at least one user, the first probability being based at least partly on an additional topic that is not explicitly associated with the at least one user, calculating for each of the plurality of users a respective query-independent second probability that the user will provide a satisfactory answer to the asker, wherein the second probability is based at least partly on a degree of social indirection between the user and the asker, determining a score for each of the users based at least in part on their respective first and second probabilities, and defining a list of candidate answerers based on ranking users of the plurality of users based on respective scores, the list of candidate answerers comprising one or more candidate answerers; and routing the query to the one or more candidate answerers of the list of candidate answerers.