1. A method for measuring advertising effectiveness for advertisements external to a social networking system, the method comprising: for each of a plurality of users of the social networking system: maintaining demographic and behavior information associated with the user, and transmitting a social networking system cookie to a client device associated with the user of the social networking system; building a log of user advertising exposures outside of the social networking system by, for each of the plurality of users exposed to an advertisement, creating a log entry indicating that the user has been exposed to the advertisement; and building a control group of unexposed users of the social networking system, the control group selected by comparing the demographic and behavior information between selected unexposed users of the plurality of users with at least a subset of the users exposed to the advertisement; wherein at least one exposed user and at least one control group user are provided a survey to determine the effectiveness of the advertisement.