1. A method comprising: at one or more computing devices comprising one or more processors and memory storing one or more computer programs executed to perform the method, performing operations comprising: determining a plurality of numeric representations for each pair of content items, of a plurality of pairs of content items, according to a plurality of similarity rules applied to the each pair of content items; for the each pair of content items, of the plurality of pairs of content items, determining a link density based at least upon the plurality of numeric representations determined for the each pair of content items; and forming a cluster comprising the content items of one or more of the plurality of pairs of content items, based at least upon the link densities determined for the plurality of pairs of content items.