What is the benefit of using this extra step (dollars -> crypto -> dollars)? What would I, as a potential user, get that I don't already get from a credit or debit card?
Maybe not much... except not needing to use your credit or debit card. Or perhaps being able to link this to one of those cards to have another method of payment via this method.
There are lots of people around the world to may not have or use those cards, or they may not have a bank account. This could give those people another option to pay for things. I can also see this solving some issues with cross-border payments and currency exchanges. (not that I do it, but I am sure a lot of people do)
I don't use FB and don't trust them, but I do use WA... was using it before they were bought by FB. And I know that it is HUGE in India. I think in this scenario, FB is the frontend for the payment, whereas companies like Visa/MC are providing the payment network. Hopefully they will keep it safe and secure, but I can't see trusting FB with anything else.