I still don't see how darkcoin falls into a ponzi scheme...

http://www.azleg.state.az.us/FormatDocument.asp?inDoc=/ars/44/01731.htm&Title=44&DocType=ARS

"Pyramid promotional scheme" means any plan or operation by which a participant gives consideration for the opportunity to receive compensation which is derived primarily from any person's introduction of other persons into participation in the plan or operation rather than from the sale of goods, services or intangible property by the participant or other persons introduced into the plan or operation.

In order to run a Masternode, an investor needs to buy 1000 DRK/DASH as collateral. These coins remain in control of the investor, but he needs to put them into one of his addresses and associate that address with the node.  The network checks that the node is running and that the funds are not moved, this classifies as a valid Masternode and then it is eligible to get paid.  With each mined block on the network a Masternode from the active Masternode list is selected and gets a payment, when all masternodes on the list are paid, the payment cycle starts again.

DASH ROI

You can estimate like this:

- Currently Masternodes get 40% of the block reward.

- The minimum block reward in DASH(DRK) is 4.65 DASH at this moment. I will assume difficulty is high enough so that we are dealing with the minimum reward for a worst case scenario situation, sometimes there are blocks with higher rewards. The minimum DASH block reward decreases by 7% annually to avoid abrupt block halvings, that already happened earlier this year so 4.65 DASH will be valid until next year.

- Average block-time in DASH is 2.5 min, so there are ~576 blocks per day. So 576*4.65DASH*40% = 1071.36 DASH alloted for the whole masternode network currently, we need to divide this by the 2300MN active masternodes. This gives us

 ~Monthly masternode income = 13.97 DASH
 ~Current Minimum DASH Annual ROI = 17%*