INTRODUCTION Online social networks, or social networking sites (SNS), have been a feature of the web since 1997, with the founding of SixDegrees.com. SNS are websites that allow users to “(1) construct a public or semi-public profile within a bounded system, (2) articulate a list of other users with whom they share a connection, and (3) view and traverse their list of connections and those made by others within the system” . More and more SNS target people who define themselves by communities according to geographic location, sexual orientation, belief systems, ethnicity, education, and countless other social attributes . Online information, advocacy, and support organizations oriented to specific medical diagnoses were among the first communities of Internet users , and SNS for patients are now also a part of the web landscape–making “community” the “killer app in health care” . In fact, the Pew Internet and American Life Project identifies 39% of US “e-patients” as users of social networks, particularly users aged 18-29 , which implies a long potential lifespan for this trend. For example, PatientsLikeMe  hosts patient communities in 16 varied diagnostic categories, including approximately 5% of all amyotrophic lateral sclerosis (ALS) and primary lateral sclerosis patients in the United States [7, 8]. This SNS incorporates not only a bulletin board, but also clinical tools. Community members report symptoms to find other “patients like them”; “tagging” of symptoms becomes useful data “emergent from shared information” .