Its guaranteed, but the problem is quality. Getting quality members from such adverts have a probability of 0.1 i.e. 10 out of every hundred or 1 out of 10. Depends on the type of site though. If you try PTC, you'll get active guys from Paid-To-Sign-Up as they are anyways trying to earn cash. Other sites/forums/etc.. totally depend on users' needs and interests though..