The exposure flows from data released by the UK parliament Digital, Culture, Media and Sport Committee's inquiry into 'fake news'.

The DUP ads were arranged by "digital advertising, web and software development" company AggregateIQ, a Canadian firm with oblique connections to Cambridge Analytica. Adverts such as seen up to 4.7 million times by Facebook users in England, Scotland and Wales, but a mere 860,000 times in Northern Ireland.

Social media firms neither publisher nor platform, we need new term - MPs
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"Social media companies cannot hide behind the claim of being merely a 'platform', claiming that they are tech companies and have no role themselves in regulating the content of their sites," the report said.

"That is not the case; they continually change what is and is not seen on their sites, based on algorithms and human intervention.

"However, they are also significantly different from the traditional model of a 'publisher', which commissions, pays for, edits and takes responsibility for the content it disseminates."

The new category - which the MPs want to see defined by government in a white paper due in autumn - should establish "clear legal liability for the tech companies to act against harmful and illegal content on their platforms". This should include content that has been referred by users and content they identify themselves.