Dark social can be defined as follows:
Dark social is a term coined by Alexis C. Madrigal, a senior editor at The Atlantic, to refer to the social sharing of content that occurs outside of what can be measured by Web analytics programs. This mostly occurs when a link is sent via online chat or email, rather than shared over a social media platform, from which referrals can be measured. (Technopedia)
Dark social is basically any traffic you get to your site because your analytics can't correctly identify the site sending you traffic. (Hubspot)
The use of and sharing through technologies like email and chat are inherently social despite the fact that they are hard to measure and not part of your site or an official social platform like LinkedIn, Twitter or Facebook.
Despite the difficulty of tracking dark social, you can evaluate your web analytics and attribute much of the referral traffic that cannot be measured directly to the sharing of a specific article or landing page as dark social. If traffic is coming directly to a non-standard page other than main navigational URLs such as your home page or a simple URL like jobs, it is likely a shared link. And, as if often the case, this attribution can be rather significant.
The other aspect of dark social that cannot be measured is when two or more people are viewing a piece of content on the same device. We have all watched a video or webinar around a company projection or 2 on 1 around a computer. It is also common to find something of interest, grab the URL and add it to an internal social platform like Slack or Jive with some commentary. Although not transparent to the originator directly, this is a valuable means of social sharing.
Dark social is a powerful means of social business and directly impacts marketing across the content lifecycle. Dark social can create more interest, awareness, consideration, purchase and loyalty for prospective and existing customers.