1. A computer-executed method comprising: receiving, by metadata storage of an advertisement manager system, first data describing characteristics associated with users; receiving, by the metadata storage, second data listing a set of sites and a plurality of links between the first and second data; for each site, computing by an analysis server of the advertisement manager system, based on the links between the first data and the second data specifying that a set of the users had an interaction with such site, a quantification for the site resulting from transformation of different combinations of characteristics, which are associated with the associated set of users including any of gender, age, or income range, into different feature values; identifying, by the analysis server, clusters of sites by applying prescribed criteria to the quantification computed and stored for each site in the metadata storage, wherein each cluster, of the clusters of sites identified, includes a grouping of sites that have similar quantifications; after identifying clusters of sites, receiving, by an advertisement server of the advertisement manager system, a notice of an opportunity for ad delivery to a current site being accessed by a current user using a user device; responsive to the notice of the opportunity, selecting, by the advertisement server, at least one ad, from an inventory of ads in advertisement storage of the advertisement manager system, for delivery to the current site being accessed by the current user based on a predicted performance of the at least one ad at the current site, said selecting the at least one ad includes: identifying, from the stored identifications of clusters of sites multiple clusters that contain the current site; computing confidence measures for the multiple clusters, each of the confidence measures quantifying a confidence of the current site's presence in each cluster of the multiple clusters; determining performance history measures of the at least one ad at the multiple clusters, each of the performance history measures specifying how well the at least one ad has performed across the plurality of sites and users of each cluster of the multiple clusters based on the linked first and second data of each cluster; computing a weighted average of the performance history measures with the confidence measures being weights; and computing the predicted performance of the at least one ad at the current site based on the weighted average, wherein the at least one ad is selected for delivery to the current site based on the weighted average; and transmitting, by the computer system, the selected at least one ad to the user device for display to the current user at the current site.