1. A computer implemented method comprising: storing action data in an online system, the action data describing actions performed by users of the online system; for each action, storing a unique object identifier and a unique user identifier, both associated with the action in the online system; retrieving, from storage, a unique object identifier associated with a selected object; using the retrieved unique object identifier to retrieve, from storage, action data describing actions in connection with the selected object that were performed by users of the online system and communicated to at least one additional user; determining, from the retrieved action data, a total number of unique user identifiers of users who have performed actions associated with the selected object and have communicated to at least one additional user about the actions performed, the unique user identifiers uniquely identifying users interacting with the selected object; generating a metric associated with the unique object identifier based on the number of unique user identifiers of users determined, the metric indicating the total number of unique users who have communicated about the selected object over a period of time; storing the metric and an association between the metric and the unique object identifier; and reporting the metric to a user viewing information about the selected object.