I have questions regarding PPC site, is this true or not?

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I did some Pay per click sites before but I was never paid, I tried doing NEOBUX, and DB-Bux but I was never paid even if I am able to withdraw the required amount, it will just suddenly turned into web page not available, I dont know with others if this they have really earned with it. Please tell me if this sites are really paying or not? Thank you...