The rise of social media--content created by Internet users and hosted by popular sites such as Facebook, Twitter, YouTube, and Wikipedia, and blogs--has brought several new hazards for medical professionalism. First, many physicians may find applying principles for medical professionalism to the online environment challenging in certain contexts. Second, physicians may not consider the potential impact of their online content on their patients and the public. Third, a momentary lapse in judgment by an individual physician to create unprofessional content online can reflect poorly on the entire profession. To overcome these challenges, we encourage individual physicians to realize that as they "tread" through the World Wide Web, they leave behind a "footprint" that may have unintended negative consequences for them and for the profession at large. We also recommend that institutions take a proactive approach to engage users of social media in setting consensus-based standards for "online professionalism." Finally, given that professionalism encompasses more than the avoidance of negative behaviors, we conclude with examples of more positive applications for this technology. Much like a mirror, social media can reflect the best and worst aspects of the content placed before it for all to see.