@Beachslim If we take the definition of money to be "a medium of exchange" then we have to concede that what you have called "fake" money is actually money. It is being used as a medium of exchange. We can differentiate money only by degrees of how "sound" it is in fulfilling the purposes that money is supposed to fulfill. We must also differentiate what a currency is over money. It could be that a currency is being used as a medium of exchange but that does not make it money because money has other functions than as a medium of exchange. Generally, currencies have in the past been money substitutes such as bank notes which are receipts to claim money. They are cancelled once redeemed so are only "current" for a short term, thus they are called currencies. Paper fiat currencies are just that currencies; currencies that were backed by money but have become backed by governments or central banks instead of money. Those paper fiat currencies now functioning as money are not as sound as some other forms of money that we have had, such as gold and silver. Coined gold and silver has been used as a currency and is also money itself but it tends to go out of circulation as it gets worn so is re-minted but the coining of that form of money was for a further convenience in trade which is one of the basic functions of money - to facilitate trade. It meant that you didn't have to carry a scale around to weigh the money.

The basic point is to differentiate forms of money by degree of soundness and from what a currency is. A paper fiat currency is being used as money but functions more as a currency than money.

@Beachslim Yes. One of the functions of money is it being a store of value. All the functions of money are variable and this is what contributes to the degree of its soundness as a medium of exchange. The more stable it is as a store of value the more sound it is as a form of money. If a medium of exchange has no value in a year it is a poor form of money and likely will not be used broadly in an economy as money. If its value holds for a hundred years it will be used more broadly but if it always holds or increases its value it is very sound as a form of money.

@Beachslim That referred more to the shelf life of a totally socialist state.
A paper fiat currency, because it is a human contrivance and under the control of an individual or individuals will probably not last long at all. The French Assignat, the American Continental, the Lincoln Greenback, only lasted for less than a decade or more.

The American Federal Reserve Note or FRN has lasted quite a long time - 58 years as you say.
Why? Because people had confidence in the integrity and honesty of the men who printed it. That's one reason. The other is because it is backed by oil. Essentially, an international petrodollar but only because by international law oil can only be bought in American dollars.

It is about to fail, as the confidence and trust of the the US government erodes and factions are starting to buck the international law of oil only being bought in American dollars. The Euro and the Yuan want to replace it.