If they are creating their won coin for their payment system for facebook then it means that they will not accept other coins or payment processors for facebook. Or if they will accept other payment processors or bank cards then it will mean that they will choose other payment processors and banks for fiat users and their Libra for crypto users.
I don't think that as per the existing laws, it is possible. They can't force the users to accept one particular mode of payment. That goes against the anti-monopoly regulations. They have to make other options available, else the users can take them to the court. On top of that, Facebook has long term partnerships with many of the payment processors such as PayPal and Visa. So even if Libra comes online (which looks very unlikely as of now), they won't be able to remove the other options.