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, each action associated with a user identifier and an object identifier; retrieving a selected object identifier associated with a selected object; determining a total number of unique 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 identifiers uniquely identifying users interacting with the selected object; generating a metric associated with the selected object identifier based on the number of unique 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 selected object identifier; and reporting the metric to a user viewing information about the selected object.