Q: Is Bitcoin a scam? What about a Ponzi scheme?
A: Bitcoin is not "scam" . Bitcoin - BTC, look like currency or asset - not as activity or any organization. Obviously, it is one of cryptocurrency which is most popular for make anonymous online payment. You can earning Bitcoin by mining, buy/trade, working, etc. So that, Bitcoin can make profitable or not is depend on your decision.
Ponzi scheme is the funding system who join after pay for who join before at crowdfunding or some collect funding format like that, Ponzi scheme does not creat any economic value, the person running it using the money and cash from people sent to people with high interest, and when they cannot paid, the system fall -> bankrupt -> Who join system after lost all their investment.

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My answer maybe so obviously, hope it is enough.