The billionaire investor says their days are numbered.
At his annual dinner at the World Economic Forum in Davos, Mr Soros harshly criticized Facebook and Google, warning that social media platforms were "obstacles to innovation".
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However, he predicted their days were numbered because tax policy and regulation would catch up with them.
Referencing Google and Facebook several times during his speech, he said: "It takes a real effort to assert and defend what John Stuart Mill called 'the freedom of mind'.
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He cautioned that this would have "far-reaching political consequences", and had already played a large role in the election of Donald Trump.
Mr Soros said the EU, which has no internet giants of its own, was best placed to "protect society against them", whereas US regulators, he maintained, were too weak.
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Facebook declined to comment, while Google did not immediately return a request for comment.