the problem is that you're responding to a substantive criticism by identifying a formal distinction. to most people making the claim, it doesn't matter whether bitcoin is literally a 'ponzi scheme' in some platonic, classical sense. some even recognize the distinction and point out that it operates as if it were a ponzi scheme, albeit through a new and interesting process. for example, eric posner's article at slate does this: http://www.slate.com/articles/news_and_politics/view_from_chicago/2013/04/bitcoin_is_a_ponzi_scheme_the_internet_currency_will_collapse.html

the bitcoin technology is itself not a ponzi scheme or any kind of scam. the promotion of bitcoins with language like 'it always goes up' or 'get in before it's too late' is, as posner argues, essentially indistinguishable from a ponzi scheme even if there is no organized head and even if the beneficiaries are distributed. the speculatory value of a bitcoin depends on the hope of selling it to others at a profit even though the coin itself, unlike a stock or bond, never generates its own productive return or dividend. (for the record, yes, gold is the same way. it doesn't matter.)