About 70 millions of people owning Bitcoin. I can´t even imagine how many people could use Facebook coin when starts.
So it would be pretty easy to overpower Bitcoin.
I think all projects after Bitcoin have a big ambition to defeat Bitcoin but in reality nothing has gone beyond Bitcoin, Even though Facebook has a very large user, I'm sure most of them don't use Facebook coins.
If we take into account that the majority of users on Facebook uses this resource only as a social network, then I very much doubt that all Facebook users will want to use this resource as a payment system. Of course, quite a lot of people use Facebook for their business purposes, but this number is still not the majority.