Ask yourself this question, If I had a magic Bitcoin engine that would double my money, why would I even tell anyone else about it? I would just use it myself and stay under the radar. It is not a Ponzi scheme, it is a scam. If you use these sites, you deserve to lose some money. It takes a bit of common sense to cure your greed.
Avoid these sites at all cost, but if you consider doing it, rather dump it in a charity. ^hmmmmm^
I mentioned that earlier in this thread:
https://bitcointalk.org/index.php?topic=1356016.msg13814064#msg13814064One can argue, as 2-3 others did that just by participating in these programmes
you are a scammer also since you know that if you get paid it will be by another losers deposit.