Facebook is better! It's easier to reach low income groups that are hungry to earn easy cash. Well, maybe not easier but this will cost less than what it would cost with adwords. What you need is attractive promotion tools that are very promising and easy to under. And a landing page that is very easy and userfriendly.
Promoting ptc sites as a member may be a little expensive unless you are very good at everything mentioned above but for ptc owners this should be a little easier.