If anyone else is wondering what that "dark social" in the chart is:
> […] social shares that do not contain any digital referral information about the source.
> In contrary to the sharing on social networking service like Facebook, which is done publicly, Dark social is done privately through IRC channels, emails, SMS or simply copy-and-paste and other ways of private sharing.
> […] social shares that do not contain any digital referral information about the source.
> In contrary to the sharing on social networking service like Facebook, which is done publicly, Dark social is done privately through IRC channels, emails, SMS or simply copy-and-paste and other ways of private sharing.
(https://en.wikipedia.org/wiki/Dark_social_media)