Despite the fact that our ability to track and measure almost every aspect of user traffic has never been stronger, there has been a strange gap in our knowledge for some time — a phenomenon that Alexis Madrigal, then with The Atlantic, referred to a couple of years ago as “dark social.” In a nutshell, it was traffic — in some cases a substantial amount — that couldn’t be identified. According to an update from Madrigal and web-measurement firm Chartbeat, most of that traffic turns out to be coming from Facebook’s mobile apps.

In the original piece that Madrigal wrote for The Atlantic, he noted that according to analytical tools like Chartbeat, more than half of the website’s social traffic was coming from somewhere other than the usual suspects — namely, the top social platforms such as Facebook and Twitter — but where exactly it was coming from…