1. One or more computer storage media having computer-executable instructions embodied thereon, that when executed, cause a computing device to perform a method for maintaining a user inventory for digital advertising, the method comprising: receiving a request for user data associated with a user identification; determining that user data associated with the user identification is not available; requesting from a user identification mapping service a list of alternative identifications corresponding to the user; sorting the list of alternative identifications to identify a match to the user identification; and associating the match with the user identification.