According to Reuters[1], "Facebook said the latest change does not have tax implications." If they are still paying taxes through Ireland for the profit they make off those 1.5b users, they seem to be choosing to EU jurisdiction (which would include the GDPR) regardless of any contract of adhesion they forced users "agree" to.
Facebook could have made a startegic decision to permanently repatriate profits based on US tax law changes before making the GDPR decision, which would make the GDPR decision tax-consequence free.
It's so the non-EU users won't have any rights under the GDPR. All non-US users were officially being served by Facebook Ireland instead of Facebook US, which would have put them under GDPR too (like India, Australia, etc.).
I don't see it in a quick scan, but I wonder if that means all the revenue from those countries will no longer be shielded from US taxes, which might indicate how seriously FB see the GDPR as a threat.
EDIT: pdkl95 mentions in another comment, they claim it doesn't.
[1] https://www.reuters.com/article/us-facebook-privacy-eu-exclu...