In 2018, TripAdvisor said users posted 66 million reviews, and its fraud-detection technology rejected 2.1% of those, or about 1.4 million reviews -- with about three-quarters of those blocked before they posted.

The company said fraudulent reviews are generally one of three types: biased positive reviews, biased negative reviews or paid reviews. TripAdvisor's content moderation team rejected or removed an additional 1.7 million reviews for guideline violations such as an incorrect location.

In total, TripAdvisor's combination of technology and human assessment rejected 4.7% of reviews in 2018 (3.1 million) either before or after they were posted.

Related: Fake TripAdvisor reviews push ‘world’s best’ hotels up the rankings