Still around 2 years after this article has been written. How do you distinguish between a legitimate business and a ponzi scheme ? In both some people spend time and effort. In a Ponzi close to 100% of time and effort is spent on getting more people to invest fiat money in the scheme. In a legitimate business a part of those involved in the scheme spend time on "something else", while the sales and marketing teams spend time and effort persuading other people to invest fiat money in the "something else" that is the outcome of the time and effort of their colleagues.
If we see this as an almost continuous scale, it would look like Steemit is closer to a legitimate business than to a Ponzi, as the vast majority of the people involved spend time and effort "producing content" and reading and upvoting content produced by others.