Online communities have contexts that are just as real, but they have the digital discontinuities we all know and love, so the odds of doing the equivalent of dropping into that work meeting after several beers is much more likely and happens far more frequently.
A separate issue is preservation - of course all this is on Your Permanent Record. And the future only has very different, limited interest in what the context is now.
I don't get the notion that social cooling was scoping itself to only the US, but even if it was, I don't think people are "learning better yet." Quite the contrary.
It's interesting that this is about big data generally and people are reading it as "Facebook".
The total amount of posting doesn't go to zero because not everybody is in the careful category.
I don't know whether the rest are silent lurkers like me, or whether they are bots, or FB is just inflating the numbers, but they surely aren't visibly active. I imagine it's not a special thing where I live, so you'll likely have the vast majority not posting publicly at all while a small minority is extremely loud. If the user count grows (hasn't the growth slowed?), most of it will not be seen by you while you look at posts and comments.
I would have thought this is obvious, but clearly not.