Wow, they're charging you $3 a month? My bill is only 10 cents a week. They must not like you or something
I don't honestly know why some forums have more involvement than others. I know one of the strongest factors seems to be personal interest and involvement in the purpose of the forum.
I visit several local websites for mountain residents. One is always active and the others are sporadic. The one that is always active was created during the 2003 fires to help people communicate with each other and share information. We all forged common bonds on that site during the first six months. Many of us have made friends with people we otherwise wouldn't have known. There are hundreds of us that check that site daily. The other sites don't have that history, and they aren't active, and they have a dozen or so participants. People go to the best and most active source for information about something they're passionately interested in. It kind of feeds on itself. You're more likely to join into a conversation with hundreds of people, than a conversation between two people.