1. A method comprising: receiving an advertisement template from an advertiser, the advertisement template comprising advertising content and one or more fields for a user endorsement; receiving targeting criteria associated with the advertisement template; providing a user interface for receiving a user endorsement in connection with the advertisement template; receiving a user endorsement for the advertisement template from an endorsing user of a social networking system; generating, by a computer system, an advertisement by combining the user endorsement and the advertisement template; receiving from the endorsing user a definition of a set of candidate viewing users in the social networking system who can view the advertisement; selecting, by the computer system, a viewing user from the set of candidate viewing users to view the generated advertisement, the selecting based on the targeting criteria, a relationship between the viewing user and the endorsing user in the social networking system, and the definition of the set of candidate viewing users; and sending the generated advertisement for display to the selected viewing user.