It's hardly a global currency when hardly any place around the world accepts it. We've got a long way to go before it achieves this title but it certainly has the potential but it's not going to happen over night. And USD isn't a global currency either. Sure, there are many countries that accept it or even have it as their defacto currency but I can't buy anything in my country or most places in Europe with it. In fact, I've probably got more chance of being able to spend my bitcoins over dollars.
USD is considered a global currency, because every central bank in the world keeps some reserves of it and usually import/export is being made with USD.
This, the US dollar is a global currency due to its reserve currency status, also for the fact that oil is traded in dollars and finally because you can go to some countries and pay with it.
But this doesnt mean that is accepted everywhere; in my country you can see signs in the supermarket that states very clearly that dollars are not accepted, only local currency.