Laws don't change in a matter of one or two days.True enough, but assuming they decided to pull the stunt before the new laws came into effect, which isn't until the middle of next year, the real question is whether public opinion could be shifted in so little time.
In most cases, I'd say that was extremely optimistic. However, in this case we're talking about a service used by probably a large majority of voters across Europe, often for considerable time every day and for communications and arrangements that matter to them personally. "If they don't change this, you'll lose Facebook, Instagram and WhatsApp" is sure to get some people's attention, and all of those services going dark would probably make the front page of major news outlets. There would certainly be a lot of discussion; in fact, it might be a doubly effective move, because a lot of people would immediately then realise that without those services their main ways of telling their friends about something wasn't working.
If that happens, how long do you think it will take a bunch of companies to spin up replacements for Facebook? [...] Thus, there would be a timeframe in which a huge number of new social networks would try to win over a critical amount of users, with one of them eventually emerging as the dominant one.
I'm not sure that's how things would play out, if Facebook really did go offline permanently in Europe because of this. It would take vast resources to operate a social network on the scale of Facebook, but more importantly, before you could even start, the first thing you'd have to do is figure out how to comply with the same EU data protection rules and presumably then you'd also have to convince some serious investors that you could do it. If Facebook had already failed in that -- because obviously Facebook isn't just going to surrender the entire EU market and all its advertising revenue without a very good reason -- then why would we expect any new social network without all of Facebook's advantages to be able to comply if it was otherwise operating on the same basis, i.e., free to use but ad-supported?