32. A system for targeting advertisements to users, the system comprising: at least one computer system; a computer-readable storage medium storing software modules for execution by the at least one computer system, the modules comprising: a criteria collection module for: receiving explicit criteria from a plurality of advertisers; providing search results to each of the plurality of advertisers responsive to the explicit criteria; receiving feedback from each of the plurality of advertisers about the search results; determining implicit criteria about which users the plurality of advertisers wish to target based on the feedback received from the plurality of advertisers, the implicit criteria indicating hidden preferences of the advertisers identified from the feedback from each of the plurality of advertisers; the criteria collection module for: receiving explicit user criteria providing information about each of the users, the criteria providing information about which advertisements the users should view; providing the users with search results using the explicit user criteria for one or more searches conducted by each of the users; receiving user feedback regarding the search results; determining implicit user criteria based on the feedback provided by the users; a profile creation module for creating a plurality of advertiser/ad profiles each using the explicit and implicit advertiser/ad criteria for a given advertiser/ad; the profile creation module for creating a plurality of user profiles each using the explicit and implicit user criteria for a given user; a profile comparison module for comparing the advertiser/ad profiles and the user profiles to generate relevance scores and determine matches between a particular advertiser/ad profile and a particular user profile; an ad targeting manager for determining which of the advertisements to present to each of the plurality of users based on placement factors including computing relevance of each of the advertisements to each of the plurality of users, computing value of each of the plurality of users to each of the plurality of advertisers based on the relevance of each of the advertisements to each of the plurality of users, and computing a price for presenting the advertisement to each of the plurality of users; and the ad targeting manager for presenting to each of the plurality of users the advertisements for which a positive determination to place the advertisement is made.
32. A system for targeting advertisements to users, the system comprising: at least one computer system; a computer-readable storage medium storing software modules for execution by the at least one computer system, the modules comprising: a criteria collection module for: receiving explicit criteria from a plurality of advertisers; providing search results to each of the plurality of advertisers responsive to the explicit criteria; receiving feedback from each of the plurality of advertisers about the search results; determining implicit criteria about which users the plurality of advertisers wish to target based on the feedback received from the plurality of advertisers, the implicit criteria indicating hidden preferences of the advertisers identified from the feedback from each of the plurality of advertisers; the criteria collection module for: receiving explicit user criteria providing information about each of the users, the criteria providing information about which advertisements the users should view; providing the users with search results using the explicit user criteria for one or more searches conducted by each of the users; receiving user feedback regarding the search results; determining implicit user criteria based on the feedback provided by the users; a profile creation module for creating a plurality of advertiser/ad profiles each using the explicit and implicit advertiser/ad criteria for a given advertiser/ad; the profile creation module for creating a plurality of user profiles each using the explicit and implicit user criteria for a given user; a profile comparison module for comparing the advertiser/ad profiles and the user profiles to generate relevance scores and determine matches between a particular advertiser/ad profile and a particular user profile; an ad targeting manager for determining which of the advertisements to present to each of the plurality of users based on placement factors including computing relevance of each of the advertisements to each of the plurality of users, computing value of each of the plurality of users to each of the plurality of advertisers based on the relevance of each of the advertisements to each of the plurality of users, and computing a price for presenting the advertisement to each of the plurality of users; and the ad targeting manager for presenting to each of the plurality of users the advertisements for which a positive determination to place the advertisement is made. 33. The system of claim 32 , wherein the advertisers are job providers interested in placing advertisements before job candidates and the users are job candidates interested in conducting searches for job openings, the advertiser/ad profiles each indicating preferences of different job providers in job candidates to fill a job opening and the user profiles indicating preferences of the job candidates in a job.