You might be able to call it a speculative (digital) commodity bubble. And there's a possibility for this "commodity" to one day go "poof!" and disappear (gov't regulation, hackers, system failure, etc.) Only time will tell us those things.

But I don't see how you could call it a pyramid scheme.

http://en.wikipedia.org/wiki/Pyramid_scheme

Bitcoin does share some aspects with a pyramid scheme. Bitcoin owners have incentive to recruit into the Bitcoin network. Bitcoins aren't really good for anything until merchants willing to accept Bitcoins also join the system. And the gain of the early adopters does come from what the later joiners bring, in the form of their goods and services that they exchange for Bitcoins.

I guess where Bitcoin diverges from a pyramid is that there's no chain reaction of recruiting. Once a user spends a Bitcoin, it's done; he doesn't stand to continually gain further from what the Bitcoin recipient continues to do or from anyone else the recipient brings into the system.

Just so be clear... How exactly does this differ from any monetary system out there? A currency is not accepted until a major of people approves it. Government or not, "money" is and will always be something that human kind has invented, it does not contains any real value, it just makes it easier to trade goods.

Bitcoin marketers invariably use formal analogies to suggest that Bitcoin isn't a scam. "The early adopters benefit just like in Google, so there can't be anything wrong with that" or "If Bitcoin is a pyramid scheme, then US dollars are as well."

The problem is that these analogies neglect all context and substance. Just because some schemes that benefit early adopters aren't scams doesn't mean that all such schemes aren't scams. Just because not everything called a "currency" isn't dishonest doesn't mean that this one isn't being marketed dishonestly and without regard to the people who will lose money as a result.

What's different between Bitcoin and gold? Bitcoin has very limited history, whereas gold has a much deeper history. Gold doesn't face existential security problems; Bitcoin has faced security compromises (such as an integer overflow in the block chain) even in its short history, and these compromises have had to be corrected using out-of-band mechanisms. The Bitcoin market is tiny, subject to market manipulation, and operates through unregulated and probably illegitimate exchanges. Finally, people buy gold without most people who own it spending a good chunk of their time promoting it dishonestly to others.