I agree with @PytagoraZ in terms of defining the thin wall between MLM and Ponzi Scheme, they are two different business model. If we look on the difference of the two, the Ponzi Scheme relies solely on the entry of future members while Multi Level marketing earnings depend on the products sold personally and commission on the product sold by the recruited person or downline. Ponzi Scheme is an illegal business model while MLM being product-focused make it a legal business.
Anyway, this is a good comparison between MLM and Ponzi Scheme, I believe it is better to hear or know it from someone who is really into the system:
https://www.linkedin.com/pulse/ponzi-scheme-vs-network-marketing-whats-difference-tim-salesIn all fairness, to compare MLM and Ponzi as synonymous is probably wrong. They aren't exactly the same banana. It's one thing to say MLM is Ponzi and quite another to say Ponzi uses MLM.
So, yeah, I agree, there is a thin wall-- not even a wall actually-- that differentiates the two. But that simply means they aren't exactly the same.
The problem, however, is that for Ponzi to become legal, to become a legitimate business, it oftentimes transforms into MLM or use the MLM model. In which case, 99.9% of MLM businesses are now essentially Ponzi. But, again, that means to say they aren't the same.