It found 50 profiles, pages and groups on Facebook, Twitter and Instagram offering stolen credit-card details, and Netflix and Uber Eats accounts.

And much of the content had remained on the platforms after being reported.

Facebook and Twitter said such activity was not tolerated and would be removed.

The investigation, carried out before the coronavirus lockdown, found one Facebook post revealing a Yorkshire man's:

• full name
• date of birth
• address
• mobile phone number
• credit-card number, security code and expiry data
• bank name and sort code

Image copyright Getty Images

Some Uber Eats accounts were also for sale on social media

According to Which?, the post had been live for four months.

Which? said it had reported it to Facebook but the social network had refused to remove it because it did not breach its community standards.

Only after Which? had requested a review of that decision had the post been removed - and, even then, the group in which it had been posted had remained active.

In response, Facebook, which also owns Instagram, told BBC News it had now acted to take down all the content.

"Fraudulent activity is not tolerated on our platforms and we have removed the groups and profiles flagged to us by Which?... for violating our policies."

"We continue to invest in people and technology to identify and remove fraudulent content and we urge people to report any suspicious content to us so we can take action."

Scam tactics

On Twitter, investigators found fraudsters offering:

• the full credit-card details of someone with a "£13,000 plus balance" for £100 - or three sets of card details for £200
• a fake passport for £3,000

Which? said it had found the content simply by searching for slang terms for fraud.

And Twitter's algorithms had then even suggested similar accounts via its "Who to follow" section.

Twitter said it was against its rules "to use scam tactics to obtain money or private financial information".

"Where we identify violations of our rules, we take robust enforcement action," it said.

"We're constantly adapting to bad actors' evolving methods and will continue to iterate and improve upon our polices as the industry evolves."

All accounts provided to it by Which? have now been suspended.

Which? Money editor Jenny Ross said: "It's astonishing that social media sites make it so easy for criminals to trade people's personal and financial information, particularly as fraud is such a prevalent crime that can have devastating consequences."

She called on Facebook and Twitter "to take stronger action to prevent their sites becoming a safe haven for scammers" and "work with the financial industry and police to address serious flaws with their platforms".