1. A method comprising, by one or more processors associated with one or more computing devices of a social networking system: maintaining, by one or more of the processors, a profile for each of one or more users of the social networking system, wherein the profile for each user comprises information about the user and information identifying connections to one or more other users of the social networking system; tracking, by one or more of the processors, actions taken by one or more of the users of the social networking system on third-party websites by: receiving one or more tracking pixel messages generated by a browser client associated with one or more of the users of the social networking system, respectively, the browser client having accessed a tracking pixel associated with a third-party website having a different domain than the social networking system, each tracking pixel message comprising information about an action taken by one or more of the users of the social networking system on the third-party website, wherein each tracking pixel message points to an endpoint at the social networking system, and wherein each tracking pixel message is generated based on a tracking identifier, a hash of the tracking identifier, a category of the action, and a value associated with the category of the action; validating each tracking pixel message by validating the tracking identifier of the tracking pixel message using the hash value; identifying a user of the social networking system associated with the tracking pixel message by accessing a user identifier associated with the tracking pixel message, the user identifier corresponding to the user; and recording, for each validated tracking pixel message, the action taken by the user on the third-party website in a pixel-user association table; correlating, by one or more of the processors, each of the actions on the third-party website recorded in the pixel-user association table with one or more social advertisements presented to one or more of the users of the social networking system, wherein each social advertisement presented to a particular user of the social networking system comprises information identifying an action taken by one or more other users of the social networking system on the third-party website, wherein each of the one or more other users are connected within the social networking system as friends to the particular user to whom the social advertisement is presented; and generating a tracking report for display to an advertiser associated with the one or more social advertisements, the tracking report comprising statistical data that associates one or more user profile attributes to one or more of the social advertisements and actions on the third-party website that correlate to respective ones of the social advertisements.