To promote the most active users we've come up with the "Users Points". When you invite your friends and they sign-up you'll get most of the points. In addition when you post, vote, etc... you get additional points. And when you delete stuff you lose the points. Profiles with the most points will be promoted to be a mod, or will get some additional privs. Well, this is just an idea for now anyway 
Please check out our list of most active users here
Comments are welcome...