Collaborative mediated environments compete to provide visitors with social feedback, whose actual effects on visitors' behavior is poorly known. This study considers feedback based on Social Network Analysis (SNA) and assesses whether this feedback is able to affect user activity in an online collaborative game. The results show that SNA feedback is able to modify group activity beyond a mere novelty effect, especially on the dimensions of the behavior covered by the feedback itself. The results also point to the possible role of task type in accounting for the feedback effect on behavior.