1. A computer-implemented method for identifying the value of bidding on an advertisement to a user, the method executed by a computer system, and comprising: maintaining a plurality of lift curves associated with a plurality of advertisers, wherein the lift curve for each advertiser identifies, relative to that advertiser's conversions, a sequence of behaviors, each behavior associated with the relative likelihood of conversion for the incremental users reached by each behavior tuple relative to an average; maintaining, for each advertiser, advertising flight information including an estimated value to the advertiser for a conversion and an average conversion rate for the advertiser; receiving a request for a placement of a advertisement from a user device, the request including a persistent identifier, and a context for the placement; identifying a set of behaviors associated with the persistent identifier; selecting advertisers to bid on providing an advertisement to the user device in response to the context of the placement; determining the lift for each of the selected advertisers relative to the set of behaviors associated with the persistent identifier, wherein the lift is determined by identifying a first behavior tuple in a sequence corresponding to behaviors exhibited in the set of behaviors associated with the persistent identifier; determining for each selected advertiser a placement value for the placement of an advertisement to the user device, using the advertiser's lift model, the advertiser's estimated value for a conversion, and the average conversion rate for the advertiser; determining advertiser bids using the placement value for each selected advertiser; selecting an advertisement from an advertiser providing a highest bid; and providing the selected advertisement to the user device.