No, I don't think facebook coin will be a threat to any crypto, why?
because facebook coin will be used only internally for facebook related platform like facebook, live, messenger, facebook games, whatsap etc.
So it will not be a direct threar to any other cryptos
In the initial use it does not look like a threat, but when we see this in a long period of time, it is very likely that global coin will have a far wider existence so that global coin might be used on a much wider platform. Facebook is a big name that can give a tremendous impact in building a real Global Coin image.