What's interesting about this is "Skymiles" are a form of private currency. Yes, there are many private currencies in use.

Banknotes also used to be private money. Each bank issued their own. The government eventually made that illegal, but banks still issue private currencies in the form of:

1. personal checks

2. cashier's checks

3. traveler's checks (though I think Amex stopped printing them)

4. credit cards

5. debit cards