1. A system, comprising: a processor; and a memory coupled to the processor, wherein the memory comprises program instructions executable by the processor to: determine one or more proximate network sites within a given proximity of a particular network site on one or more network navigation paths, wherein the one or more proximate network sites are determined dependent upon user network traffic generated by a plurality of users visiting the particular network site from other network sites, wherein the given proximity of a given network site is determined dependent on one or more of the following characteristics of the user network traffic: a quantity or type of the plurality of users visiting the particular network site from the given network site, a number of intermediate network sites visited by one or more users between the given network site and the particular network site, or an amount of time elapsed between one or more users visiting the given network site and visiting the particular network site; determine one or more keywords associated with the one or more proximate network sites, wherein to determine one or more keywords associated with the proximate network sites, the program instructions are further executable to perform at least one of: analyze content of the one or more proximate network sites; determine one or more search engine query terms associated with the one or more proximate network sites; or determine one or more keywords associated with the proximate network sites from an a keyword association store maintained by an advertising brokerage service; and associate at least one of the keywords with the particular network site.