Actually, the definition of a Ponzi Scheme is a fraudulent venture that lures investors and pays profits to earlier investors with funds from more recent investors. If they were asking us to pay some sort of membership fee from which the senior members were getting their rewards - this would be a Ponzi scheme. But when steemers choose to purchase more steam power that doesn't directly benefit those who joined before them or those that referred them. It actually benefits the buyers whose vote now weighs much more than that of a user with mere 15SP.

The best comparison is the online game. Those that are serious about succeeding (or winning?) do spend some of their money buying weapons and armor or whatever other tools they need - even though these tools are virtual and are worth nothing in the real world. Same thing here - people may wish to purchase some virtual power with real money and they may not get any material benefits from it, but this doesn't make Steemit a Ponzi scheme.