Government generally know what Facebook is , and they know how powerful the owner is financially, whatever cryptocurrency product he is creating may really not be in favor of government and the best any government can do is to start creating policies that will ban cryptocurrency. I just hope that Facebook coin will not do more damage to cryptocurrency than the benefit we think it will bring.
Where did you hear Facebook's coin will be decentralized? Is it even true? If Facebook controls the coin it's not. It's just distributing the power/control to something else (here an enterprise (FB) instead of a government). This coin will be just a centralized (digital) currency, nothing more. I don't even understand why do you use the term 'cryptocurrency' to describe it. It's not a cryptocurrency at all.
There is no need to ban cryptos, and it would be inefficient and they perfectly understood it. That's why, as for example, the head of IMF wanted to be able to track everything in the EU. After they got it that they can't, they switched their politic to "cryptos must be regulated" and this is what countries are doing since, with new local regulations, G20 dinners (only to become fat!), harassing exchanges in their business, etc...