1. A method of combining content site visitor profiles, the method comprising: under control of a hardware processor: receiving first content site interaction data from a first tag embedded within a first content page, the first content site interaction data comprising a cookie-derived first visitor identifier associated with a first visitor of the first content page; identifying a first persistent identifier of the first visitor from the first content site interaction data; receiving second content site interaction data from a second tag embedded within a second content page, the second content site interaction data comprising a cookie-derived second visitor identifier associated with a second visitor of the second content page; identifying a second persistent identifier of the second visitor from the second content site interaction data; determining that the first and second persistent identifiers are the same; and in response to said determining, aggregating the first content site interaction data with the second content site interaction data in a single visitor profile, such that the first and second visitors are considered to be the same visitor.