I don't think the purpose is to create their own money, but to create a vehicle that takes advantage of crypocurrency-like protocols.
For example, perhaps they'll let users generate their own keys, thereby letting everyone transact freely with each other. This may allow FB to wash their hands of KYC laws except for those depositing and withdrawing fiat directly.
Facebook would still be maintaining the ledger, which means there's no such thing as letting everyone transact freely with each other. It's like the bank saying they're not liable for KYC procedures because it's their customers who initiate the transactions.
For example, perhaps they'll let users generate their own keys, thereby letting everyone transact freely with each other. This may allow FB to wash their hands of KYC laws except for those depositing and withdrawing fiat directly.