1. A method, comprising operations of: receiving first data describing prescribed characteristics of users; receiving second data listing sites; for each site, based on the first data and the second data, computing a mathematical representation of the site according to the prescribed characteristics of any of the users that have conducted one or more predetermined types of interactions with the site; identifying clusters of the sites whose computed representations are similar according to prescribed criteria; responsive to notification of an opportunity to deliver unidentified advertising to a given user via a given site, selecting at least one ad based upon factors including the ad having a prescribed performance history at one or more clusters containing the given site; performing one of the following: transmitting the selected ad directly or indirectly to the given user at the given site, or transmitting a bid to deliver the selected ad to the given user at the given site; and where at least one of the operations is performed by a processor.