Facebook's a juggernaut whether we like it or trust it I think just like their stock, that their own coin would have value and be traded.
However, for someone security and privacy conscious like us would we ever buy or trade with it? The answer would be no and I think many would agree.
If we can't trust Facebook with basic privacy and security, they can't be trusted to handle our finances.
It's also important to remember that many companies have tried to knockoff PayPal with their own payment system and essentially all of them have failed.
I am really split on the issue of whether it would succeed.