1. A computer-implemented method for improving computer-implemented content selection, the method comprising: processing user activities in a social networking system through a relevancy filter to identify a subset of user activities that are relevant to a viral propagation study; in response to selecting a user activity as the graph exploration seed, constructing a user activity cascade by exploring a social graph in the social networking system, starting from a starter social network node corresponding to the selected user activity, wherein the user activity cascade comprises social network nodes found during said exploring; and determining that the user activity cascade is virally propagating, wherein the determining improves computer-implemented content selection and wherein the determining is based on one or more of: a total size of the user activity cascade, an expansion rate of the user activity cascade, a diversity measure of the user activity cascade, or any combination thereof; wherein, one or more of: said exploring the social graph includes exploring one or more social network nodes connected to one or more already explored social network nodes via one or more social network edges and wherein the social network edges correspond to one or more specific social connection types; said determining that the user activity cascade is virally propagating includes counting a number of social network nodes in the user activity cascade corresponding to user activities within a time window; said exploring includes exploring one or more social network nodes connected via directional social network edges, and wherein identifying the root network node includes exploring the social graph along one of the directions until no further social network node is found; the method further comprises: determining whether the user activity cascade is similar to another user activity cascade being monitored by a viral analysis system; and in response to determining that the user activity cascade is similar to the other user activity cascade, merging the user activity; or any combination thereof.