Social groups are just to chat with people about any subject you want.
Don't ask me why we have them, because they seem to be just like Clubs to me, and we have a whole section for clubs.
The only use I can see for them is if you have a competition game, you can make a group for it. I can't really see any other uses since you can just make a Club in the Clubs section.
But it's still fun to join! ^_^