What you need to look at is the ratio between the amount in circulation and how much is spent every year. For $1billon in bitcoin this figure is ≈1%. For dollar (I'll take MZM[1] to provide a conservative result, but it would be even higher if you took M0 or just cash), this ratio is over 120%! So really, bitcoin isn't really used as money, and mainly as a highly speculative
financial asset.
Also, $1 billion sounds huge, but for a payment medium it's still really low: worldwide, last year that's also the order of magnitude of the transactions made in Vanuatu vatu. As a comparison, PayPal recorded more than 500 billions dollars in transactions last year.
[1] https://fred.stlouisfed.org/series/WMZMNS