Yelp's CEO can deny things all he wants, but proof printed on his own website says otherwise. Below's a thread about someone giving a company a bad review, and then a Yelp employee going out of their way to contact and eventually harass the reviewer. When the reviewer blocks the employee's communications to avoid further harassment, the employee then opens a public thread, and divulges private emails.
The employee at first vehemently denies being paid by Yelp, but later gets caught confessing to the whole thing: "yelps pays me a lot of money to protect its sponsors. its a good racket. dont blow it for me." The fact that this same employee who began the harassment has also given the other company a glowing review, is completely unethical to say the least.