A method for tracking a stability of an identifier and selecting content to present on a client device associated with the identifier based on the stability of the identifier. An identifier defines how a client device accesses online systems via a network. An online system stores tentative identifiers. From the stored tentative identifiers, the online system identifies stable identifiers. An identifier's stability is based on the identifier's interactions with online systems via the network. The online system ranks the stable identifiers. The online system stores a quantity of top-ranked identifiers from the ranked stable identifiers. The online system identifies an impression opportunity for a client device associated with an identifier. In embodiments in which the identifier comprises a top-ranked identifier, the online system determines content to include in the impression opportunity based on the identifier and presents the determined content on the client device.