I hate to quote wikipedia but:
In economics, the money supply or money stock, is the total amount of money available in an economy at a specific time.[1] There are several ways to define "money," but standard measures usually include currency in circulation and demand deposits (depositors' easily accessed assets on the books of financial institutions).
This is a reasonable definition of money supply. The fact that the block chain can never contain more than 21 million bitcoins puts no limit whatsoever on how many can be in circulation (just as a bearer bond can circulate the same way dollar bills do) or in demand deposits. The reason you don't see equivalents in circulation with dollars is because of a combination of legal restrictions and the lack of any need for them.
It may be "reasonable" but it isn't used. A bearer bond isn't part of US money supply.
For example no commonly accepted definition of US money supply includes mortgages which you would include for bitcoin economy. Similarly the US money supply would be many magnitudes larger if you considered all mortgages, students loans, credit card balances, corporate bonds, derivatives, stocks, options, credit default swap, US national debt, etc to be part of the money supply. It may be part of the economic activity but it isn't the money supply.
The same applies for global money supply and bitcoin money supply.
Debt isn't used to conduct trade. Sure debt trades hands but it is the item being sold not the medium for trade.
People buy debt.
People sell debt.
However the other side of that trade normally involves money.
People buy potatoes
People sell potatoes
However the other side of that trade normally involves money.
Unless your definition of the bitcoin money supply includes potatoes it shouldn't include debt either.Nobody for example goes to Grocery stores and pays for groceries with some debt they own. The money supply is what facilitates trade not what is purchased.
Grocery <-> Currency
Debts <-> Currency
Videogames <-> Currency
...
Houses <-> Currency
The reason we track & measure the money supply is it is the intermediary that facilitates trade. No matter what someone wants (from debt to videogames) they can acquire it via money and likewise when someone wants to sell something they can sell it for money.
If you trade debt for something other than money that is barter. It doesn't make the debt part of the money supply. Likewise if I trade you $1600 in potatoes for an ounce of gold are those potatoes part of the gold supply now. Did the gold supply grow by $1600? Of course not and likewise if you barter some debt you own for some other asset it doesn't increase the money supply. I mean if you include debt in the money supply to not be arbitrary anything bartered for gold should be part of gold supply.
Lastly one trait of money is fungibility. Each dollar is worth exactly the same as every other dollar. Each unit of debt isn't worth the same (even if it has the same face value and interest). Some will default, some will payoff early, some will payoff in full. When you get paid in dollars (or bitcoins) it doesn't matter
which dollar you get paid with however if you get paid in debt it does matter which debt you get paid with.
While you can barter with debt it isn't money and thus isn't part of the money supply (any money supply).