First, the definition:
"A Ponzi scheme is a fraudulent investment operation that pays returns to its investors from their own money or the money paid by subsequent investors, rather than from any actual profit earned by the individual or organization running the operation."
Just for fun, I ran a Google search for "is a Ponzi scheme." Let's take a look at the results, for science:
1) "Social Security" is a Ponzi scheme.
2) "Facebook" is a Ponzi scheme.
3) "Suburban Sprawl" is a Ponzi scheme.
4) "Medicare" is a Ponzi scheme.
5) "Full Tilt Poker" is a Ponzi scheme.
6) "The American Dream" is a Ponzi scheme.
7) "The US" is a Ponzi scheme.
8) "Gold" is a Ponzi scheme.
9) "Real-Estate" is a Ponzi scheme.
10) "Greek Bonds" [are] a Ponzi scheme.
In short, the definition of a Ponzi scheme has grown to become "anything that acquires value only through the participation and acceptance of many," which covers basically everything except food, water, air, sunlight, and the company of others (except mothers-in-law).
All monetary systems, by definition, require the participation and acceptance of many, yet we need monetary systems to have a functioning economy. Bitcoin is just a more efficient and fair monetary system. Using this new, looser, definition of Ponzi scheme, yes Bitcoin is a Ponzi scheme, but this looser definition is neither incriminating nor offensive.