The story
From dark (“hidden, secret”) + social (“relating to social media or social networks”); compare Dark Web (“the portion of the Deep Web hosted on restricted networks”). Coined 12 October 2012 by Alexis Madrigal of The Atlantic (see quotations), in reference to Internet traffic (clicks) from hyperlinks sent via private messages that are thus invisible to most analytics programs.
Etymology from Wiktionary, CC BY-SA 4.0.