Oh, this would be a whole can of worms that Facebook would not want to open.
International transactions are very regulated in Brazil and certainly not only the Central Bank would be heavy with their hammer, but Receita Federal (tax authority) would also be very interested in this.
When PayPal started operating in Brazil, they had to convert all Brazilian accounts held in USD to BRL, and there are some restrictions on currency exchange operations that accounts registered in other countries don't have.
