11. A computer program product for determining social proximity between users of an online system, the computer program product comprising a non-transitory computer-readable storage medium containing computer program code for: accessing a profile of a user of the online system maintained by each of a plurality of social networking systems, each profile specifying a first plurality of additional users of a social networking system connected to the user via the social networking system; identifying a user of the first plurality of additional users of an identified social networking system; identifying a second plurality of additional users of the identified social networking system connected to both the user of the online system via the identified social networking system and to the identified user of the first plurality of additional users; generating a social proximity score based at least in part on a number of users in the second plurality of additional users connected to the identified user of the first plurality of additional users, the social proximity score indicating a relative social proximity of the user of the online system to the identified user of the first plurality of additional users; and recommending a content item to the user of the online system based at least in part on an interaction of the identified user of the first plurality of additional users with the content item and the generated social proximity score.
17. The computer program product of claim 11 , wherein generating the social proximity score comprises: identifying a group maintained by the identified social networking system including the identified user of the first plurality of additional users based on a profile of the user of the online system maintained by the identified social networking system; and adjusting the social proximity score based on at least one of a name of the group and a set of social networking system users in the group.