When I first signed up to Patients Like Me, there were 1100 MSers who had already signed up. There are currently more than 4900. There must be a reason. They are actively recruiting and it seems to be working.
There are communities for MS, but also Parkinson's, HIV/AIDS, and ALS with plans to expand with other conditions. Now they have added a Mood Community that already has 1700 registered.
In March, The NY Times Magazine published an article about the benefits of MSers communicating with MSers about specific symptoms and drug reactions and just living with MS. Click on the Multimedia graphic to enlarge it. It shows some community groups by gender, age, and country. This is truly an international site.