[r00fus]

It's not that people won't eventually add their debit card

Your payee has to a) have the FB app installed, b) have a debit card, c) from a US-based bank and d) agree to add that debit card to the FB app.

It's more than just "pay on FB". Enough little hurdles to present immediate adoption friction. Many people know that you can do chargebacks on CCs but can't do that for debit cards so they have reservations about adding that debit card to online transactions (never mind that FB Pay is likely just using ACH in the background).