This all comes down to Facebook and friends holding on to your money, making interest on it, while everybody passes their tokens around. Many other companies have been doing this for a very long time, just not in such a widespread way and opening their currencies up to whoever wants to use it.

Take xbox live's original currency (still in use) where you pay $X for Y coin. They had your money in the bank, while you had some coins in your account, just sitting there, doing nothing for you. They had a bit more control and you could only buy amounts of coin that were not equal to the amount of coin you had to spend to buy anything, so you always had some left in your account.

In Facebook's case, just the time it takes to convert money to Libra and then back into actual money will net them some interest on it. On such a large scale, that's a constant income.

They're trying to be a non-profit because they don't actually charge for the service, but the investors still make money by way of holding on to your cash while it's in Libra form.