IMO steem is very close to a Ponzi scheme, but it isn't one.
I've been wondering, too, where the money for the payouts is coming from. Apparently steem does not produce monetry value. Therefore, in the long run, steem cannot pay out more monetary value than money is being pumped into the system. Up to this point, the Ponzi comparison holds.
However, it seems that steem creates monetary value after all, in the form of voting power. People are buying steem and powering up in order to increase their voting power. So clearly voting power does have monetary value, and this is where the monetary value of the payouts is coming from. Therefore, steem is not a Ponzi scheme.