One of the ways that fiat currencies are accepted by the public is that you can use them to pay taxes so even if no one wants to accept them you can be sure that the issuing government will accept it as payment for taxes and government services. So people are assured that at worst there is a place where it can be used.

Once FB creates services that people want and then FB decides that they will only accept its currency as payment then people will have the confidence to accept it and use it. There is no doubt in my mind that FB will create services since all companies eventually create them as part of their business.

There is nothing new about the general idea. It's happened in the past and it will happen in the future.