Anyone tempted to #DeleteFacebook after the personal data of millions of users fell into the hands of a political consultancy is still likely to be monitored by the social network, which tracks nearly 30 percent of global website traffic.

And Google, in various guises, shadows 64 percent of all web-browsing worldwide, a recent study of 200,000 German users by Cliqz using its anti-tracking product Ghostery, said.

A larger study of web trackers by researchers at Princeton University in 2016 produced similar results, with Google Analytics and other Google trackers taking the top five places, followed by Facebook.