More specifically, no one is being guaranteed a specific (large) direct return in a specific amount of time. In a ponzi scheme the person who sells you the investment also gives you the return (from the investments of the next batch). They tell you they'll "triple your money in three months!" and they do. Until they don't.

In a ponzi scheme there isn't even a thing, commodity or otherwise, being invested in really. You never own anything except the promise of return. Your money is going nowhere but to the last batch of people to buy in via the person you bought it from, and then when Mr. Ponzi decides its time, into his own pockets as he runs away as fast as he can.

It's entirely possible to consider bitcoin a scam, or a bubble, but it really has no similarity to a ponzi scheme. It's a commodity asset that may have no long term value that's been over-zealously sold.

[Added Note: It'd possibly be accurate to describe what Mt.Gox specifically was/became as a ponzi scheme]

> over-zealously sold.

Right there.

I think the technology is great, but it has been speculated into the stratosphere.

If I invested my life savings into a company that hasn't turned a profit in five years, that wouldn't make the company or the exchange a scam, it would simply make me an idiot.

Nobody is promising Bitcoin investors a profit because nobody (officially) speaks for Bitcoin to begin with. Anyone investing into BTC and subsequently losing or gaining money is doing it of their own will, not because they were scammed into it.

You don't know what a Ponzi scheme is, that's clear.