1. A method comprising: (A) receiving, from a client computer, a request for an impression to be displayed in a placement of a web page, the request being made by the client computer as a result of said client computer receiving the web page; (B) receiving from the client computer a placement identifier particularly identifying a placement for the impression in the web page, the placement identifier having been embedded in the web page received at the client computer; (C) receiving behavioral data from the client computer, the behavioral data being indicative of client actions taken on multiple websites previously visited by the client computer and indicative of impressions previously received at the client computer; (D) determining a plurality of candidate impressions that may be displayed in the placement of the web page, based, at least in part, on the behavioral data; (E) determining a learning mode of each impression in the plurality of candidate impressions, the learning mode of each said impression being indicative of a number of times said each impression in the plurality of candidate impressions has been served to web pages in client computers on the Internet, wherein said learning mode is one of a plurality of learning modes, said plurality of learning modes comprising at least a first learning mode, a second learning mode, and a third learning mode, wherein, impressions that have been served less than a first predetermined threshold number of times are in said first learning mode, and wherein impressions that have been served more than said first predetermined number of times and less than a second predetermined threshold number of times are in said second learning mode, and wherein impressions that have been served more than a third predetermined threshold number of times are in a third learning mode, said second predetermined threshold being greater than said first predetermined threshold, and said third predetermined threshold being greater than said second predetermined threshold; (F) selecting a selected impression from the plurality of candidate impressions based at least in part on the learning mode of each of the impressions; and (G) serving the selected impression to the client computer, wherein selecting the selected impression from the plurality of candidate impressions in (F) comprises: (f1) if all impressions in the plurality of candidate impressions are in said first learning mode, then randomly selecting the selected impression from the plurality of candidate impressions; and (f2) if all impressions in the plurality of candidate impressions are in said second learning mode, then selecting a highest revenue generating impression in the plurality of candidate impressions as the selected impression; and (f3) if all of the impressions in the plurality of candidate impressions are in said third learning mode, then selecting a highest revenue generating impression in the plurality of candidate impressions as the selected impression; and (f4) if at least some impressions in the plurality of candidate impressions are in different learning modes, then selecting, as a final set of candidate impressions, impressions in the plurality of candidate impressions that are either; (i) all in the first learning mode, or (ii) all in the second learning mode, or (iii) all in the third learning mode, and then selecting the selected impression from the final set of candidate impressions.