Naively, it's going to slow because they're already saturated the market; they have a userbase of about 223 million US, which is 67% of all Americans (88% of adult Americans over 19, if we naively assume their TOS is followed and no kids have FB accounts).

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.