1. A method of generating demographic profiles of network-accessible sites, the method comprising: obtaining demographic information relating to each network user of a first set of network users, said demographic data supplied by the respective users; obtaining network usage information relating to each of the network users of the first set of network users; training a computer-implemented classifier using the demographic information and the network usage information relating to each of the network users of the first set; obtaining network usage information for a second set of users for whom demographic information is unknown or incomplete; applying the computer-implemented classifier to the network usage information of said second set of users to generate predictive demographic information for the users in the second set; and generating, by a computer system that comprises one or more computing devices, a demographic profile of a network-accessible site based on a combination of (1) the demographic information supplied by users who have accessed the site, and (2) the predictive demographic information of users who have accessed the site, wherein generating the demographic profile of the site comprises giving an amount of weight to a user's supplied or predictive demographic information that is proportional to a measure of the user's quantity of usage of the site.