The interesting thing is that this really doesn't matter that much in the grand scheme of things.
Consider weapons treaties, like the UN ones banning the use of land mines and cluster munitions. The only countries that have signed them either don't have any reason to use them, or are allied with a nation that hasn't signed that treaty.
This is much the same story. The US hasn't signed this, and never will (because it explicitly contravenes a cornerstone of its supreme law), and at that point what the other countries do is pointless unless they outright block US services from their networks- in which case there will be riots in the streets. Governments don't survive for long when they alienate the vast majority of their population, and the majority of the population uses US services.
Combine that with the simple fact that 100% effective moderation of an online service is unscalable to the point of being impossible without prohibiting any meaningful content/conversation means that countries that do sign this and implement it in their law will never be able to develop a competitive Facebook alternative, and all you've accomplished (as a signatory nation) is political posturing and shooting yourself in the foot.
You can't outcompete a free nation. That's kind of its main advantage.