A couple of months ago I created an Exfoliative Cheilitis Group on the biggest social community sites out there (the one where you usually use your real full name). So far only one person has joined.
The group is "open", which means that is is searchable, anybody can join, and the group will show up in the persons profile page. This is probably one reason why people are hesitant to join.
Would you the social community group if it was secret and invisible (i.e. do not show up on your profile page)?
If I make the group secret it will no longer be searchable and you need an invitation to join, which makes it harder for people to find it and more work for me managing members. But it is better to have a group with members, than a group without them.