There is a definition of what ponzi scheme is, and cryptocurrencies just don't fall under it even if you try to put them there. For starters, in any Ponzi there's hierarchy. There are people who created it, early adopters and those who got involved by other adopters. While there is a dev team and buyers of tokens, when you as a holder of crypto A tell your friends about it, you don't profit from it at all. So what if your friend is now also holding crypto A? The price can go up or down, and it won't matter who invited whom when it comes to profits and losses. Moreover, most of normal cryptos are distributed in a very decentralized way, while in the Ponzi the main people have access to enough product to ensure control.