1. A computer-implemented method comprising: receiving information describing interactions of users of a social networking system with a plurality of news feed stories maintained by the social networking system; identifying one or more news feed stories from the plurality of news feed stories for presentation to a viewer; for each of the one or more identified news feed stories, identifying a first set of features describing interactions of other users of the social networking system with whom the viewer has previously established a connection, the features from the first set comprising a first aggregate measure based on interactions of the other users, wherein the interactions of the other users are with objects described in the news feed story, the first aggregate measure weighing interactions of the other users based on their affinity with the viewer; training a first news feed ranking model using the first set of features, the first news feed ranking model configured to rank candidate news feed stories selected for presentation to the viewer; for each of the one or more identified news feed stories, identifying a second set of features describing interactions of the viewer, the features from the second set comprising a second aggregate measure based on interactions of the viewer, wherein the interactions of the viewer are with objects described in the news feed story; training a second news feed ranking model using the second set of features, the second news feed ranking model configured to rank candidate news feed stories selected for presentation to the viewer; and determining whether to use the first news feed ranking model or the second news feed ranking model to rank candidate news feed stories selected for presentation to the viewer based on a number of interactions of the viewer with the objects described in the news feed story being below a predetermined threshold.
2. The computer-implemented method of claim 1 , wherein the first set of features further describe interactions of the other users with other news feed stories similar to the news feed story.