A computer is useful in a way a bitcoin isn't. You can do useful and fun stuff with a computer, but all you can do with a bitcoin is exchange it for something useful. Money is just a way of keeping track of value in order to facilitate an open market. It plays a vital role, but it isn't actually a good or service.
So you agree money provides a useful service -- indirect exchange -- and thus produces real value by enabling people to be better off than if they could not conduct indirect exchanges via money. (Though you don't quite appreciate the implications thereof.)
And Bitcoin is (trying to be a generally-accepted kind of) money with advantages over other moneys.
So you already agree this is different from on Ponzi scheme in that Bitcoin provides value (or at least the honest potential to). All you have to add in response, to attempt to substantiate your position, is that you're personally dismissive of the benefits. And that certainly is your right, just as others are every day finding genuine use cases for it where it really does better than other currencies. (And many think Macs are crap and everyone using them is just deluded.)
But it does not at all substantiate your claim that Bitcoin is some "Ponzi scheme", wholly dependent on more people putting more money in while producing no valuable product.
Indeed, Bitcoin adoption could stop right now without a single person "holding the bag" as in a Ponzi scheme -- for example, if it stabilizes as the medium of exchange for a niche market (*cough* online illegal drugs) in which people prefer to swap in/out of Bitcoin (from/to another currency at a varying exchange rate) than try to trade using a "standard" medium like USD.