Aren't they subject to GDPR because they do business in the EU with EU citizens? I don't see how that would change.

Probably, but not in the same way. I'm guessing they wouldn't have been fined for external data transfers if their servers were located outside the EU.

It could be that the EU would block them if they wanted to host fb/etc... including ads, and didn't have a presence in the EU, but I'm not sure if that's the case.