There are circumstances, however, when there is no referrer data. You show up at our doorstep and we have no idea how you got here. The main situations in which this happens are email programs, instant messages, some mobile applications*, and whenever someone is moving from a secure site ("https://mail.google.com/blahblahblah") to a non-secure site (http://www.theatlantic.com).
This means that this vast trove of social traffic is essentially invisible to most analytics programs. I call it DARK SOCIAL. It shows up variously in programs as "direct" or "typed/bookmarked" traffic, which implies to many site owners that you actually have a bookmark or typed in www.theatlantic.com into your browser. But that's not actually what's happening a lot of the time. Most of the time, someone Gchatted someone a link, or it came in on a big email distribution list, or your dad sent it to you.
And then one day, we had a meeting with the real-time web analytics firm, Chartbeat. Like many media nerds, I love Chartbeat. It lets you know exactly what's happening with your stories, most especially where your readers are coming from. Recently, they made an accounting change that they showed to us. They took visitors who showed up without referrer data and split them into two categories. The first was people who were going to a homepage (theatlantic.com) or a subject landing page (theatlantic.com/politics). The second were people going to any other page, that is to say, all of our articles. These people, they figured, were following some sort of link because no one actually types "http://www.theatlantic.com/technology/archive/2012/10/atlast-the-gargantuan-telescope-designed-to-find-life-on-other-planets/263409/." They started counting these people as what they call direct social.
The second I saw this measure, my heart actually leapt (yes, I am that much of a data nerd). This was it! They'd found a way to quantify dark social, even if they'd given it a lamer name!
On the first day I saw it, this is how big of an impact dark social was having on The Atlantic.
Just look at that graph. On the one hand, you have all the social networks that you know. They're about 43.5 percent of our social traffic. On the other, you have this previously unmeasured darknet that's delivering 56.5 percent of people to individual stories. This is not a niche phenomenon! It's more than 2.5x Facebook's impact on the site.