A site entitled PatientsLikeMe states its goal very clearly - this is a site for sufferers to share experiences and views, and with 240,000 registered users (or 220,000, the site lists two different totals) it clearly meets a need. How does it rank as an example of a community site?
<--break->I started by looking for a condition, which is where I imagine most users would start. I keyed Alzheimer's, and the resulting screen started with a definition of the disease, followed by a table:
Table of symptoms