Premium publishers made approximately 14% of their ad revenues from distribution platforms like Facebook in 2016

"Publishers are receiving far less money than might have been expected from placing their content on the third-party distribution platforms owned by companies including Facebook, Google, and Snapchat, according to a new report.
The report, from premium publisher trade body Digital Content Next (DCN), claims that the (mean) average premium publisher generated $7.7 million in revenue from distributing their content on third-party platforms in the first half of 2016 — equivalent to around 14% of their overall revenues in the period."

Source:  Business Insider, 24th January 2017