A facility for Performing on-line advertising at a subject web site relative to a selected user activity is described. The facility receives a plurality of bids. Each bid is submitted on behalf of an advertiser, and specifies an advertising rate that the advertiser will pay to display an advertising messages to users performing the selected activity with respect to the subject web site. When a request for any web page of the subject web site containing advertising space is received from a user that has previously performed the selected activity, irrespective of the identity of the requested web page, the facility both (1) identifies the received bid specifying the largest advertising rate, and (2) causes an advertising message to be displayed in the advertising space of the requested web page for the advertiser on whose behalf the identified bid was submitted.