13. A method comprising: accessing a database of advertisements, each advertisement associated with targeting criteria that specifies targeting the advertisement to a user who is connected in a social networking system to another user who has interacted with a particular object in the social networking system; identifying a viewing user to receive advertising; identifying one or more other users who are connected to the viewing user in the social networking system; identifying one or more candidate advertisements from the database of advertisements using the targeting criteria; selecting an advertisement from the candidate advertisements for display to the viewing user; generating by a processor a social story based on an interaction between a user from the identified one or more other users who are connected to the viewing user and the particular object in the social networking system; and sending the selected advertisement and the social story for display to the viewing user.
13. A method comprising: accessing a database of advertisements, each advertisement associated with targeting criteria that specifies targeting the advertisement to a user who is connected in a social networking system to another user who has interacted with a particular object in the social networking system; identifying a viewing user to receive advertising; identifying one or more other users who are connected to the viewing user in the social networking system; identifying one or more candidate advertisements from the database of advertisements using the targeting criteria; selecting an advertisement from the candidate advertisements for display to the viewing user; generating by a processor a social story based on an interaction between a user from the identified one or more other users who are connected to the viewing user and the particular object in the social networking system; and sending the selected advertisement and the social story for display to the viewing user. 15. The method of claim 13 , further comprising: generating an index for each of at least some of the users with who the viewing user is connected, the index mapping each user to one or more objects with which the users have interacted; and using the index to identify a plurality of objects in the social networking system with which one or more other users have interacted.