This research explored the persuasive effects on behavior of social feedback by a robotic agent. In two experiments, participants could save on energy while carrying out washing tasks on a simulated washing machine. In both experiments, we tested the persuasive effects of positive and negative social feedback and we compared these effects to factual feedback, which is more widely used. Results of both studies indicated that social feedback had stronger persuasive effects than factual feedback. Furthermore, results of both studies suggested an effect of feedback valence indicated by more economic behavior following negative feedback (social or factual) as compared to positive feedback. Overall, the strongest persuasive effects were exerted by negative social feedback. In addition, results of Experiment 2 indicated that task similarity increased the persuasive effects of negative feedback. The implications for persuasive robotic agent theory and design are discussed.