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.

The same network effect that allows social networks to grow at a blistering pace also work backwards.

It seems to me that some people and the mainstream media want to see Facebook in crisis, so they interprete any stats or analysis with such bias.

I can't tell, are those monthly active user numbers for all Facebook owned services, or just Facebook itself?

If everyone quit Facebook and replaced it with Instagram, that's a very different story.