Selecting social endorsement information for an advertisement for display to a viewing user

A social networking system provides an advertisement for display to a viewing user along with a social endorsement, which contains information about a friend of the viewing user in the social networking system. The endorsement is selected from the social information maintained by the social networking system. To select the endorsement, the system identifies social networking system objects related to the advertisement as well as interactions between these objects and other users who are connected to the viewing user in the system, where each interaction relates to a candidate social endorsement. An affinity score may be computed for each candidate social endorsement, where the affinity scores represent a measure of the viewing user's likely interest in each candidate social endorsement. The system selects a social endorsement and provides it for display to the viewing user with the advertisement. In one embodiment, the selection is based upon the computed affinity scores.