Why we are not Ponzi? I already wrote how it works. Let me give you an example:
Investor deposited $100 in BitCoins. From this amount we take ex. $30 and these we invest in mining performance. It remains $70 ... At one point, the price of Bitcoin $230. We will buy the $ 70's. Moments later it cost $265. But we have a part of mined bitcoins. In the finals we gain too. It's simple math.
This is only model situation. In fact, the numbers are much more interesting...