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Re:Why we don't sell ads – WhatsApp Blog by tim333 on Monday April 20, 2015 @12:37PM (#9409229)
It is more likely that you visited a site with 'Facebook social integration' on it.

>"When a logged-in Facebook user visits a site with Facebook social plug-ins, Facebook receives the Facebook ID and browser ID, along with the URL of the page being visited,"

quote from a The Verge article:

http://www.theverge.com/2015/3/31/8319411/facebook-tracking-...

Re:Why we don't sell ads – WhatsApp Blog by spyder on Monday April 20, 2015 @02:28PM (#9409927)
He said he is using ABP with "Fanboy's Social Blocking List" which is blocking these social plug-ins.