Steem may not be a Ponzi in the strict sense, however...

Steem distributes wealth without acquiring any. I really don't see how this can work out in the long run. When on July 4th 2 million SBDs have been distributed and everybody tries to cash out the whole scheme will simply collapse. In order to pay out the rewards, there must be demand for STEEM, and there's really no reason to buy any. The only reason I can image is to increase your voting power, and voting power is mostly good for generating income. And that's a point where we're suddenly VERY close to a Ponzi.