Facebook and eBay have vowed to prevent fake reviews being posted on their sites.
The social media network and the online auction have both signed agreements with the Competition and Markets Authority in the UK, in which they have promised to "better identify, investigate and respond" to misleading reviews.
The CMA urged them to watch out for the reviews which were being sold to the likes of Amazon and Yelp.
In a statement, Facebook said: "Fraudulent activity is not allowed on Facebook or Instagram, including offering or trading fake reviews.
"While we have invested heavily to prevent this kind of activity across our services, we know there is more work to do and are working with the CMA to address this issue. This includes exploring the use of automated technology to help us detect and remove this content quickly, before people see it and report it to us."
As a result, Facebook has closed 188 groups and removed 24 accounts, whilst eBay has banned 140 users.
eBay's spokesperson said: "We maintain zero tolerance for fake or misleading reviews and will continue to take action against any seller that breaches our user policies. We welcome today's CMA report, as well as their acknowledgement of our ongoing enforcement work on this issue."
CMA chief executive, Andrea Coscelli, added:
"Fake reviews are really damaging to shoppers and businesses alike.
"Millions of people base their shopping decisions on reviews, and if these are misleading or untrue, then shoppers could end up being misled into buying something that isn't right for them - leaving businesses who play by the rules missing out.
"We're pleased that Facebook and eBay are doing the right thing by committing to tackle this."