Interesting, had no idea, the site does a poor job of "advertising" this.
Here's a suggestion: use their wonderful
pop-up ad feature to very occasionally post a "Hate ads? Become a sponsor today and never deal with them again".
I cannot imagine I am in the vast minority who simply have a quick link/favorite to the CBJ board, almost never venture on to other boards, never bother with their profile page or glance at the ads. All I do is read other people's posts and post themselves.
I wonder which teams have the most "sponsor" users. If the site wanted to get people to become sponsors they could hold a competition to see which team can get the most signed up.