(Going off topic, I know.) It's a less-egregious substitution, but Ponzi != Pyramid in some important ways. If they were the same, we wouldn't have needed to invent different terms. :)

    1. Actual Fraud
     * A Ponzi Scheme requires lying to people about what assets currently exist, or how much of the asset is reserved as "theirs". 
     * A Pyramid Scheme just asks people to collectively speculate (without guarantee) that tangible profits will arrive.
    2. Layers
     * A Ponzi Scheme can be one criminal and an indefinite number of unwitting victims. [0]
     * A Pyramid scheme (like the structure) requires *growing interior layers* of participants that blur the line between victimizer and victim.

[0] Insofar as some Ponzi Scheme targets do not suffer losses, that's not because they were necessarily complicit, but because the criminal strategically chose to let some marks exit in order to maintain the illusion.