A centralized currency can not mean real competition for bitcoin, since taking into account that it will be available to Facebook users, it is a fact that privacy will never be a factor in our favor. In addition, Facebook is facing many challenges in terms of its decline in popularity and its tendency to technological monopoly.
Maybe some speculators will be able to invest in that coin looking for some economic benefit, but those same investors will always have bitcoin as a safe haven and as a preferred cryptocurrency.
Global coin could be centralized coin but in investment market, investor must be not too care about centralized or decentralized coin because the matter thing is the potential in the future. If facebook launch their own cryptocurrency and used in all facebook platform, it could be a big deal for investor because facebook platform used by most people in the world.