Facebook remains flooded with fake review groups, despite being ordered to take urgent action by UK regulator Competition and Markets Authority (CMA), the study found.
"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, Which? Head of Products and Services, 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," Hitchins added.
Which? found dozens of groups on the social networking site in the UK that are recruiting people to write fake or incentivised reviews, with sellers offering free products in exchange for highly-rated reviews for products listed on Amazon.
During the investigation researchers joined ten of these Facebook review groups and found 3,511 new posts generated in just one day, and more than 55,000 posts over a 30-day period.
The true overall figure could well be higher as Facebook caps the number of posts it displays, Which? said.
In June, UK's Competition and Markets Authority warned Facebook and eBay to conduct an urgent review of their sites after it found "troubling evidence" of a thriving marketplace for fake online reviews.
The platforms were told to remove and prevent these groups from reappearing.
While eBay seems to have largely eradicated listings offering five star reviews for sale, Facebook continues to be full of fake review groups, the research found.
The rise in fake reviews could increase the chance of people potentially being duped into buying poor quality or even unsafe products that have been boosted by disingenuous reviews.
A Which? survey of the public showed that 97 per cent of people use online when researching a purchase.
"Facebook must immediately take steps to not only address the groups that are reported to it, but proactively identify and shut down other groups, and put measures in place to prevent more from appearing in the future," Hitchins said.
( With inputs from IANS )