Earlier quoted context omitted.
They are absolutely required to report this to the data protection agencies in all European countries. As the other comment mentioned, missing the 72 hour deadline on this is enough to get a fine as Booking.com did. I'm curious to see the total in GDPR fines from this for Facebook. Will probably take a year or two before we know.
GDPR is a gift to large corporations. Regulatory capture in return for a slap on the wrist. It also burdens startup competition and trains people to click "Allow Cookies" and "Accept the Terms of Service" as fast as possible.
The actual work involved is trivial if you minimize data collection, which is the whole point - you shouldn't collect anything you don't actually need and GDPR got rid of the "abusing user privacy is purely profitable" excuse.
Regulatory capture is an anti-piracy bill that requires scanning all uploads using technology that only a few companies have or that costs more than potential income of a business. That's why YouTube was generally pro-that bullshit EU "anti piracy" law.