TripAdvisor.com — knowingly allows fake reviews, deceiving the visitors to the website and defrauding hotel owners
The short version:
Tripadvisor has a review system which allows anyone to post anything about any hotel, regardless of whether they have even stayed there. This allows all sorts of people to abuse the system.
An example: a hotel was blackmailed and told that if they didn't give a former guest some money, the guest would write a false malicious review to harm the business. Tripadvisor would not take down the false review nor would they allow the hotel management to respond properly to the review. The result is that anyone reading the review will get a false idea of the hotel and the hotel will lose bookings as a result.
Also, their copyrighted slogan "get the truth" is a complete lie. How is it possible to get the truth from a review system which is full of holes?