1. A method comprising: receiving, at an online system, third party hashes for a plurality of targeted users, each hash generated from a third party unique identifier of a corresponding targeted user; generating local hashes for one or more local users of the online system, each local hash generated from a local unique identifier of a corresponding local user, the local unique identifier of each local user maintained by the online system; identifying as matched users those local users with local hashes that match those of the third party hashes corresponding to one of the plurality of targeted users; generating, by the online system, one or more inferred identifiers for each of the one or more local users excluding the matched users, the inferred identifiers being of the same type as the local unique identifiers of the one or more local users, the inferred identifiers generated based on characteristics of each corresponding local user as maintained by the online system; identifying as inferred matched users at least one of the local users that have local hashes of corresponding inferred identifiers that match a third party hash of a third party unique identifier of one of the plurality of targeted users excluding the matched users; providing, to a third party system, a first selection including the matched users and a second selection including the inferred matched users.