500 bits is attractive offer for just sign up. How much can we earn in your forum?
You didn't say how much do you pay per post.
There were many other similar forums that pays their users for posting but too bad most of them failed to get many active users.
To low payment per post perhaps?
Actually paying out per post is quite unstable business model, no any websites can make profit without large number of daily active members so if any forum want to pay their user in pay per post basis they have to do it with their own pocket money. How long an owner can pay like this way if he failed to attract new daily traffic and advertisers?