Of course that's not true. All you must do is find a counterparty willing to take Facebook stock as payment for Amazon stock. (I am not aware of any exchange listing such a contract, but it's certainly possible over the counter.) And if you do, you will have to pay taxes on the gain and will have a new cost basis in your Amazon stock.
With a few exceptions (like-kind exchanges come to mind), you owe the capital gains tax when you dispose of the property. That you continue to take on market risk afterwards does not enter into it.
With a few exceptions (like-kind exchanges come to mind), you owe the capital gains tax when you dispose of the property. That you continue to take on market risk afterwards does not enter into it.