As a business owner, I have also seen the extortion for advertising model. Businesses that advertise with Yelp have low star reviews filtered, while businesses like mine that do not, have reviews from valid customers (often with friends and other reviews) filtered, to lower star ratings.

I have also seen reviews that are blatantly fake (e.g. reviews from Santa Claus, comical reviews, etc.) persist, until a significant amount of time passes. This indicates manual removal, and no actual Yelp filter.