Dark Social is somewhat less sinister than it may sound. In a basic sense, it refers to sharing on social media that is unable to be tracked accurately, i.e. the stuff that isn’t picked up by web analytics programs.
How it works is that if someone were to click a link directed to your site from what is labelled as an 'open' social media platform such as Twitter, Facebook, Instagram or LinkedIn, your resulting analytics will in theory show you where that click came from.
The idea of 'Dark Social' is derived from people increasingly sharing links through private messages using the likes of Facebook messenger, Snapchat and WhatsApp. Also, the continued sharing of links through emails and text messaging.
It is as simple as copy and pasting a link you find interesting to a message and pressing send. An action that millions of people perform every day. However, because links that are shared in this way do not have referral tags, the visit to the linked page is lumped in the 'direct traffic' category (despite not really being direct traffic at all). But we can’t really expect analytics programs to tell the difference, right?
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.