Yeah that happens on many forums, even some that are more active than this, people make groups and no one joins, or even if they do there's no point in posting in the group when you can post in the main forums for everyone to see.
It's kind of like Facebook pages, sure we "like" them and follow their updates, and maybe even comment on a status, but if you and one of your friends both like the page of a tv show, you aren't going to go to that show's page to talk about it, you're either going to message each other, or post on each other's wall