1. In a network environment that includes at least an advertisement computer system, a method for targeting online advertisements to users based on the probability that a user will respond to possible advertisements, comprising the following: an act of accessing a plurality of possible advertisements; an act of determining a response probability for each advertisement in the plurality of possible advertisements, the response probability representing a probability that a user will respond to an advertisement; and an act of altering a score associated with each of the possible advertisements based on the response probability for each of the possible advertisements, the score being used to indicate whether or not to present the advertisement to the user.