Leaving the EU would have very strong network effects and is IMO not an option for Facebook. The entry barrier to social networks is so low now that this could trigger a mass exodus in the rest of the world.
They are doing ok AFAIK, but are almost two orders of magnitude smaller than FB, user wise. I don‘t think Xing is even doing that well, they have started very aggressive monetization tactics recently, like forcing people to purchase a pro account in order to view incoming friend requests.
I don't know about _strong_ network effects, but part of the appeal of Facebook (for me) is that it's universal - visitors to the country (e.g. exchange students) and people I've met overseas (holidays) are all in one place. Further, it doesn't result in you being introduced to (possibly better) Facebook alternatives in those situations. It may not a strong effect, but there is definitely small cross-country effects IMO.
Leaving doesn't mean erasing all the profiles. It just means shutting down local corporate presences. It'd suck for the employees but Facebook can run things out of Palo Alto just fine.
I'm not sure what it'd involve for contracts and payments. But many EU firms have US legal presences too. They could easily buy ads on Facebook through their US presence. Multi-nationalism works both ways.
The GDPR does not allow international players to store data on EU citizens without conforming to the rules. So if FB leaves in order to avoid conformance, then they have to delete all EU citizen data.
You are right, this might be the case, someday.. but as long as they have their tax-free money parked in the EU, removing all legal entities from the EU will be very expensive..
From the perspective of the US company, then it's not them getting fined so not their problem. And if the EU wants to fine its own companies for buying adverts, they can go right ahead and do that. Doesn't seem very practical for the local economy though.