In Techcrunch, they point at RareShare, a a community site for people with rare medical condition. I will briefly quote part of the article:

 ”shared some of it most recent growth data with us and the results are impressive. Since launching over the summer, the site has grown each month by between 35%-50%, and now has over 700 communities associated with different disorders. Given the nature of the site, the numbers are bittersweet, but it’s nice to know that so many people are finding others they can connect with”.

To me, this is very similar to what I was saying about Health 2.0 recently, the potential of social networks for people with particular health conditions is still unleashed. The article also notes that communities can gather in standalone sites of a specific disease, but RareShare is a starting point for an umbrella.