For each of various sites, a mathematical representation is computed according to prescribed characteristics of users that have conducted one or more predetermined types of interaction with the site. Cluster of the sites are identified 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, at least one ad is selected based upon factors including the ad having a prescribed performance history at one or more clusters containing the given site. The selected ad is transmitted to the given user at the given site, or a bid is transmitted for such placement of the ad.
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