UK consumer group Which? has found that there are hundreds of groups on Facebook recruiting people to write fake, highly-rated reviews on Amazon in exchange for free products and other incentives.

A UK regulator found "troubling evidence" of a thriving marketplace for fake online reviews on Facebook and eBay and asked the companies to shut these down.

While eBay seems to have some success with this, according to Which?, Facebook has not. Facebook did not immediately respond to Business Insider's request for comment.

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Facebook has become a breeding ground for fake Amazon review groups — and it's not doing enough to prevent it, according to UK consumer group Which?.

Which? found that there are still hundreds of groups on Facebook that are recruiting people in the UK to write fake, five-star reviews on Amazon in exchange for free products and other incentives.

The investigation comes just weeks after the Competition and Markets Authority (CMA), a UK antitrust authority, told Facebook and eBay to conduct an urgent review after finding "troubling evidence" of a thriving marketplace for fake online reviews on their sites.

The two companies were told to remove and prevent these groups from reappearing. But while eBay seems to have some success with this, Facebook hasn't, according to Which?.

"Our latest findings demonstrate that Facebook has systematically failed to take action while its platform continues to be plagued with fake review groups generating thousands of posts a day," Natalie Hitchins, head of products and services at Which? said in a statement.

"It is deeply concerning that the company continues to leave customers exposed to poor quality or unsafe products boosted by misleading and disingenuous reviews."

In a statement emailed to Business Insider, a Facebook spokesperson said that the company has removed nine of the 10 groups that Which? reported and that it is investigating the remaining group.

"We continue to improve our tools to proactively prevent this kind of abuse, including investing in technology and increasing the size of our safety and security team to 30,000," he said.

After joining the 10 groups, Which? found that more than 3,500 new posts were added each day or more than 55,000 over a 30-day period.

Once the researchers joined a group, they were also referred to other similar ones by a Facebook algorithm. "This made it unnervingly easy to find more and implies a possible weakness in the algorithm," Which? wrote.

Which? also saw big increases in the number of members in certain groups, indicating that when one group closed, members just flocked elsewhere.

It is now urging the CMA to take action against Facebook to prevent customers from being "duped" into buying "poor quality' or "unsafe" products in the future because of these disingenuous reviews.

"The CMA must now consider enforcement action to ensure that more is being done to protect people from being misled online. Which? will be monitoring the situation closely and piling on the pressure to banish these fake review groups," Hitchins said.