An online system receives third party user identifying information. The online system accesses accuracy measures associated with each of a plurality of the user identifying information sets. The online system identifies high accuracy sets of user identifying information that include the one or more types of user identifying information included in the received types of the third party user identifying information. The online system identifies as high confidence matches those local users of the online system having the high accuracy sets of user identifying information that match a corresponding set of third party user identifying information for the plurality of third party users. The online system also identifies as low confidence matches, and identifies as similar matches those of the low confidence matches that have a measure of similarity to one or more of the high confidence matches that is beyond a threshold measure of similarity.