As for international money transfers: TransferWise and the like are already providing better solutions in that space without forcing you to use Facebook Coins. Now you might say that mostly caters to the rich world, and that's true, but the people who can't use TransferWise are probably not going to be able to use Libra/Diem either for the same reasons they can't use TransferWise: ID documents, KYC/AML, plus capital controls and sanction regimes which aren't magically going to disappear for this new currency.
It won't provide one of the few benefits of Bitcoin etc either, namely the ability to make illegal transactions more easily.