chris, seems to me you could even run a Ponzi scheme with a single investor. For instance:
Take in $1 million, pay the investor back lump sums of $100,000 after 12 months , $110,000 after 24 months, and $121,000 after 36 months. Tell him that he's been earning 10% annually on his investment. Then abscond with the remaining $669,000.
[Granted, this is mathematically not much different than skipping on an unsecured loan].
While in a pyramid scheme, the entire sales pitch to "investors" depends on a hoped-for exponentially increasing number of suckers.