Abstract: An ad facilitation server receives a definition of an audience from an advertiser, the definition expressed in terms of a URL that audience members have visited, such as a regular expression for URLs. Upon receiving a URL corresponding to a visit of a visitor to a page of the advertiser, the ad facilitation server determines whether the URL matches the audience definition provided earlier; if so, the ad facilitation server sends a request to a number of different ad publishers, requesting them to add the visitor to an associated audience. When an ad impression opportunity occurs on a particular ad publisher, the publisher considers bids previously submitted by the advertiser for the audience when selecting an ad for the ad impression opportunity. Thus, advertisers are enabled to bid on specific audiences defined by prior page visits, across a number of different ad publishers constituting different advertising channels.