6. The computer-implemented method of claim 1 wherein each referral includes a query text, a query string, and a source identifier.
1. A computer-implemented method for generating advertisement sets based on analysis of external referrals to a web site, the method comprising: under control of one or more computer systems configured with executable instructions, collecting information from one or more referring sources that are external to the one or more computer systems and that refer visitors of the referring sources to the website, the information corresponding to referrals to the web site by the referring sources, and each referral comprising referral information corresponding to a request for additional content that is sent responsive to user interaction with content of a corresponding referring source and that resulted in a visit to the website referred to by the corresponding referring source; for each of the referrals, identifying a referral type based at least in part upon a unique combination of a landing page type of the referral and one or more elements selected from a referring source identifier and a product identifier, the landing page type identifying a type of landing page that is displayed in response to user interaction with the referral; aggregating the referral information for each referral type, the aggregated referral information including a financial metric for the referrals; and for each referral type, determining whether the aggregated referral information satisfies an advertising criterion; when it is determined that the aggregated referral information satisfies an advertisement criterion, generating, based at least in part upon the aggregated referral information, an advertisement set for the referral type, the advertisement set having an associated link including a query string corresponding to the referral type, each referral type having at least one corresponding query string; and submitting the generated advertisement set to an order placement service.