1. A method comprising: receiving, at a first computing device associated with a social-networking system and from a second computing device associated with a third-party system, a query comprising a first identifier corresponding to a user of the third-party system, wherein the first identifier is computed by applying a hash algorithm to a user credential associated with the user of the third-party system; determining, by the first computing device, whether the user of the third-party system matches any member of the social-networking system, comprising: for each member of the social-networking system, accessing a second identifier corresponding to the member of the social-networking system computed by applying the hash algorithm to a user credential associated with the member of the social-networking system; and comparing the first identifier and the second identifier; and if the second identifier corresponding to a member of the social-networking system matches the first identifier, then sending, by the first computing device to the second computing device, social-network information of the member of the social-networking system in response to the query.