The former deputy prime minister said the social media company had analysed its data and found no “significant attempt by outside forces” to sway the Brexit vote.
He also dismissed suggestions that Cambridge Analytica, which harvested the data of up to 87 million Facebook users, had influenced the UK’s decision to leave the EU.
Sir Nick, who was hired by Facebook in October last year, told BBC Radio 4’s Today programme: “We ran two full analyses of all the data we have in the run-up to the Brexit referendum, we’ve shared all of this information with the select committee in Westminster and elsewhere – we have found no evidence of a significant attempt by outside forces.”
Asked if he was saying it was a “myth” and a “conspiracy theory” that Facebook was exploited to the benefit of the Leave campaign, the Remain-supporting Sir Nick said: “Yes. I’m someone who was debating with Nigel Farage before Facebook was invented.
“And, much though I understand why people want to sort of reduce that eruption in British politics to some kind of plot or conspiracy or some use of new social media through opaque means, I’m afraid the roots to British Euroscepticism go very, very deep.”
The former Liberal Democrat leader, now Facebook’s vice-president for global affairs and communications, said public attitudes towards the EU had been “much more influenced by traditional media than they have by new media”.
Labour MP David Lammy, the former culture minister, described Mr Clegg’s comments as “horse manure”.
He tweeted: “What about the disinformation spread by Russian state media, RT and Sputnik, on Facebook? Hope Zuckerberg paid you a high price for any integrity you had leaving office.”
A report on disinformation and fake news published by the House of Commons Digital, Culture, Media and Sport Committee in February noted that Kremlin-aligned media published “significant numbers of unique articles about the EU referendum” and that those with heaviest anti-EU bias went most viral.
The social reach of these articles dwarfed that of content shared by the Vote Leave and Leave.EU websites, according to analysis by communications agency 89up. The value of a comparable paid-for social media campaign would be between £1.4m and £4.14m, the committee’s report said.
MPs have called for greater regulation of tech companies and Sir Nick said Facebook now agreed there was a “pressing need” for new “rules of the road” on privacy, use of people’s data, election rules, and adjudicating on what constitutes hate speech.
Asked whether Facebook should not already be addressing such issues itself, Sir Nick insisted implementing new rules was not something the company “can or should” do on its own.
“It’s not for private companies, however big or small, to come up with those rules. It is for democratic politicians in the democratic world to do so,” he said.
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