I don't think Ponzi scheme applies at all. In this case, the value and price of a share is based on the actual revenue growth of amazon from non-investor provided income. If amazon was taking on new investors to pay out money to old ones, it might apply.

It's certainly possible amazon could end failing and the stock would end be worthing much less than expected. It seems unlikely the name brand, market capture, datacenter and distribution facilities, etc would not end up with amazon being acquired by a walmart or something ala AOL/TW.