Harsh truth is that if facebook creates its own cryptocurrency, it won't be to benefit the users financially, it will be to benefit the company financially. Blockchain is the future and companies know it. However, decentralized blockchains are not favored by companies, they won't be able to benefit. The way facebook would use blockchain is the way ripple uses blockchain, it gets people to think they are benefiting themselves when in fact they are benefiting the banks. With Facebook it would probably only mean that users could send prepaid currency to eachother, which Facebook would monetize on, similar to ripple 60% facebook, 40% users.