I'm sure some do, happens in the restaurants and hotel ratings systems, too (yelp, Google, trip advisor). It would be interesting to know how often this happen

I'm sure some do, happens in the restaurants and hotel ratings systems, too (yelp, Google, trip advisor). It would be interesting to know how often this happens compared to buying the 5 star reviews. My guess would be that the fake negative reviews for competitors are less than 10% of the total fake reviews.