at worst, facebook will pull out of EU and close its irish HQ (but probably still be tracking users). Who won?
Whichever European social network replaces them.
Facebook is a freak social network considering how long it has survived. People used to migrate every one to two years.
GDPR makes it very difficult to touch user's private data for commercial purposes. that's the whole business of a SN so this is kind of precluded
If we start from scratch without all that it would be possible to make a profitable social network for a couple of bucks per month per user (or a tiered system, so heavy users or “influencers” would pay more while the base tier remains free).
They won't of course, because it leaves the market open for grabs.
Due to trade agreements US companies have to follow EU law, for EU citizens, even of those companies don't have a presence in the EU.
In other words Facebook would have to block EU users.