I have not heard of any forum where you can earn passively with your posts, the important thing is to post since these forums are made to learn and communicate with others about business, online earnings, investments, etc... so sharing information and knowledge is vital and making new contacts and learn new ways to earn money.
I also think that if you simply keep earning it could be a scheme that is not sustainable for the forum in terms of payments and rewards for users and the forum could close and go bankrupt, there are many paid to post forums that are really worth it out there, like the one we found.