1. A method comprising, by one or more computer systems of a social-networking system: receiving, at the social-networking system and from a third-party system that is separate from and external to the social-networking system, a query comprising a first identifier corresponding to a user of the third-party system, wherein the query is sent responsive to the user of the third-party system accessing the third-party system via a client device of the user, and 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 social-networking system, whether the user of the third-party system matches a member of the social-networking system by comparing the first identifier with each of one or more second identifiers that each correspond to one of a plurality of members of the social-networking system, wherein the second identifier is computed by applying the hash algorithm to a user credential associated with the member of the social-networking system that the second identifier corresponds to; and sending, by the social-networking system to the third-party system, social-network information of the member of the social-networking system corresponding to the second identifier matching the first identifier in response to the query if one of the second identifiers computed by applying the hash algorithm matches the first identifier computed by applying the hash algorithm.