The threat to Martin Lewis’s reputation was plain enough. The journalist, television presenter and website founder is in the business of giving financial advice. A series of Facebook advertisements, paid for by various parties, gave the impression that he was associated with dubious financial come-ons — bitcoin, binary trading, PPI checks and so on. He had nothing to do with any of the ads, which used his name and in some cases his image.

Mr Lewis (whose writing has appeared in the Financial Times) complained to both Facebook and the UK’s Advertising Standards Authority. Facebook took down ads, and the ASA found in his favour against the advertisers. But other, similar ads soon popped up on the social network, Mr Lewis alleges. So he is suing Facebook for exemplary damages — that is, damages high enough to encourage a change in behaviour — in the UK high court.

In Britain, publishers and advertisers are jointly liable for defamatory advertisements. This raises the familiar question of whether Facebook is a publisher at all. In the case of its users’ posts, this is a complex issue. The network has some responsibility for monitoring and policing third-party content. Legally this responsibility varies by jurisdiction, but Facebook's own policies about removing, for example, hate speech from its platform shows that it accepts this basic point.