Not much can be said about the whole healthcare social networking phenomenon now. There are plenty of sites out there, enough to make me almost give up trying to review all of them. But pioneers like MedHelp are worthwhile to talk about.
MedHelp gives the (now) standard online community features to users looking for better information and support around thier medical problems. It was started around 1994 and now draws more than 2 million unique visitors a month. Perhaps that is why they have long-standing partnerships with the top medical institutions like Cleveland Clinic and Partners Health. Not sure if advertising is the only source of revenue for them- plenty of companies willing to pay for access to this kind of rich data for patient insights.
Jan 2011 Update: Just read that MedHelp has partnered with GE to create a new mobile mood diary app that helps users keep tabs on their mood to better understand their mental well-being. The app, called MoodyMe, is available as a free download on iTunes for the iPhone and iPod touch.