What exactly has gone wrong with PTC sites ? Is it over exposure or scam?

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After what happened to neobux which according to most of us was a rock steady PTC site ,I was trying to analyse what could have gone wrong with all these sites? Since the past few months we have seen many PTC sites come , disappear and come back again with a new reduced earning capacity. Is it because the site has made enough profit and is now taking it easy or is the site going to close down for some reason or the other? Or is the whole system of online earning going through an evolution? are we going to see something new taking the place of PTC sites ? Friends what are your views ? Please share ...