In order to advance the understanding of conformity processes in online environments, this research examined how peer reactions to an ethically questionable call to vigilantism on Facebook influence the individual’s responses to this initiative. In an experiment, we varied peer reactions in form of textual comments and numeric displays (the number of people who have already “liked” or “shared” a message). While the valence of peer comments and numeric information had an interaction effect on the individual’s intention to support this call online, offline support intentions and attitudes toward the call were only affected under conditions of high identification with commenters. Dispositional altruism and empathy directly influenced individuals’ willingness to participate in this initiative of vigilantism—independently of other users’ reactions. Implications for theoretical models of conformity processes in online realms are discussed.