Losing 2 to 4 million users out of 375 million users is hardly "significant". It's something to look out for of course, but trying to insinuate that Facebook is in trouble in Europe with those numbers is a stretch.
It's definitely significant for a company that a) has grown steadily since inception, and b) has a valuation based on continued growth. Facebook's stock is trading something like 35% below this year's peak, and this is part of why.