Its actually quite simple. Using your ref link you refer members. Those members must have make atleast 10 post before they become your referral.
Contest runs till we get 500 members (or 1000, cant decide)
First winner gets $20 cash in Paypal
One person gets $20 might not be enough of an incentive. You may have to add a few levels of awards to make it more competitive and give people more incentive to refer people. After a few months you could probably reduce it back down to a single winner per week but initially you are probably going to have to offer an expanded prize structure.
I don't know that PTC is dead but it is definitely not what you want to base your whole business model on. It is a nice occasional gimmick that you can get away with using to launch your site with along with other activities and you can use it here and there but it is not something you want to bet the farm on.