1. A method of displaying an impression in a web page, the method comprising: receiving a request for an impression to be displayed in a placement of a web page, the request being provided by a client computer as a result of receiving the web page; receiving a placement identifier particularly identifying placement, the placement identifier being embedded in the web page; receiving behavioral data from the client computer, the behavioral data being indicative of websites previously visited by an end-user on the client computer and indicative of impressions previously received in the client computer; determining a plurality of candidate impressions that may be displayed in the placement based on the behavioral data; determining a learning mode of each impression in the plurality of candidate impressions, the learning mode being indicative of a number of times each impression in the plurality of candidate impressions has been served to client computers on the Internet; selecting a selected impression from the plurality of candidate impressions based on the learning mode of each of the impressions; and displaying the selected impression in the web page.