Steemit is not a Ponzi scheme, because by definition, a Ponzi scheme implies that new users are paying for the old users to cash out. This clearly isn't true. Steemit is not a scam by intent, however, there are still many questions about how it might grow and sustain.

However, there's still a real risk of the system collapsing under itself due to bad inflation and content incentivization issues. Now, are these concerns real? That's debatable. Perhaps inflation won't be a problem because the value of the network grows faster than the inflation rate. Tweaks to website itself, the flood of new users, and outside developers are already changing the meta-game of what kinds of content are incentivized on the platform. If the amount of good output greatly outweighs the amount of bad output, it is possible that Steemit can reach an escape velocity to truly take off and sustain.

This is a huge experimental system that has never been done before, so nobody really knows how this will turn out. But I'm very intrigued by the experiment and I'm looking forward to seeing how it will turn out.