How exactly might it be "a big pyramid scheme"?
Early adopters have a huge incentive to convince more people to use Bitcoin. If Bitcoin is successful the value of Bitcoins is guaranteed to increase because there's a finite number of them. Because of this, naturally people will hoard their Bitcoins rather than spend them. At some point people will realize the price of Bitcoins was driven up by speculation, and it will collapse.
I'm not saying Bitcoin was intentionally devised as a pyramid scheme, but that's kind of what it looks like it's turning into.
But I don't see how you could call it a pyramid scheme.
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.
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.
A pyramid scheme is a non-sustainable business model that involves promising participants payment, services or ideals, primarily for enrolling other people into the scheme or training them to take part, rather than supplying any real investment or sale of products or services to the public. Pyramid schemes are a form of fraud. (Wikipedia)
promising participants payment, services or ideals
There's an implicit promise that if Bitcoin gains popularity the value of Bitcoins will increase.
primarily for enrolling other people into the scheme or training them to take part
If you take part by purchasing Bitcoin there's a huge incentive to get other people involved in Bitcoin.
rather than supplying any real investment or sale of products or services to the public
There are very few services/products you can actually buy with Bitcoin.
Hence I would argue that it can be argued that Bitcoin is a bubble (i.e. equivilent to investing in a startup that is sure to fail) but not a pyramid scheme.
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It's not a traditional pyramid scheme, but I see a lot of similarities between that description and the current state of buying/selling stocks.
promising participants payment, services or ideals
There's an implicit promise that if a stock gains popularity the value of the stock will increase.
primarily for enrolling other people into the scheme or training them to take part
If you take part by purchasing stocks there's a huge incentive to get other people involved in the stock market.
rather than supplying any real investment or sale of products or services to the public
There are very few services/products you can actually buy with a stock certificate.
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Bitcoin is a bubble, but not necessarily a scam.
Even if no market exists to resell a stock, if the company is operating it still has fundamental value, whereas something like bitcoin has no value if nobody else is willing to trade you something.
Given that, I don't think your comparison holds. Yes, a speculative market exists in stocks (and generally, anything else of value.) But the value of a stock is not based purely upon the speculative market that might exist around it - at its root, it has an underlying fundamental value that is not driven by market demand.
Similarly, lots of folks speculate in commodities, but at the end of the day, if you can't sell oil or steel, you can use them to produce other things you can sell.
You can argue that currency is a kind of stock in the "enterprise" of the sovereign printing that currency. But is there a qualitative difference between that and bitcoin?
It is in fact the opposite of a claim on the printing operation of the issuer, given that the more currency is printed, the less valuable the currency you hold becomes.
If nobody will accept a currency, it does not continue to have value, because its value is based upon what someone is willing to trade you for it.
Market demand for the instrument, that is, right? :)
I don't know -- I see a difference here, but it looks only nominal.
If I purchase shares in a privately-held small business that hits tough times, my claim on the output of that company can essentially evaporate -- they won't be paying dividends as no profit exists, I might not be able to sell the stock because no market exists for it, and once the company goes belly-up they pay the bank and lenders before shareholders get anything.
I'm sure a difference exists; I'm no serious investor, so it's easy for me to not see it.