From the beginning of June 2015 Facebook is bringing more referral traffic to publishers than Google:

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Mobile is drastically decreasing the number of … searches … You don’t search when you’re on mobile, you open apps… When you’re in front of a computer, practically every action in your browser begins with a search. On mobile? Not so much. It’s slow and inefficient to write stuff, that’s why apps exist. One tap and you’re done. The combination of these two behaviors (mobile discovery vs. desktop search) generates probably the biggest threat to Google ever, given that the bulk of its revenues come from search… Facebook’s secret plan to kill Google… By Armando Biondi, AdEspresso

See also: Facebook Just Declared War on Google By Sam Biddle.