A Ponzi scheme is an investment operation where the founder pays returns to initial investors with the funds from new investors, and fraudulently keeps this fact from investors. So Bitcoin is not a Ponzi scheme.

It may, however, be a pyramid scheme, where early adopters can only profit by getting others to invest at ever-higher prices.

History will either record Bitcoin as a currency or a pyramid scheme, and anyone who says they're sure which one it'll be is either a fool or a liar.

Yes, but only in the same sense that every currency that has greater face value than the value of its physical ingredients is a pyramid scheme.

... and in the same sense that most things humans do are [pyramid] schemes :-)

The US dollar is backed by Americans. A bit coin is backed by who?

Bitcoin users. Which is a larger population than that of many countries that have national currencies.

The phrase "backed by Americans" is very hand-wavy.

Earthians, gaians, world citizens ;)

Including me.

Humans with Internet access