An outsize number of Yelp reviews are the work of a small but vibrant community of consumers. This so-called elite squad writes about half of the site's restaurant reviews, according to a study by Michael Luca, an assistant professor at Harvard Business School.

Being an elite Yelper is like joining an exclusive, invitation-only club. Elites can attend what Yelp calls its “epic parties,” which the firm sponsors with local businesses, such as breweries and restaurants. In some cases, elites have received tickets to Broadway-style shows, courtesy of Yelp. “We don't get financial compensation, but it's a great perk,” says Eric Rosenberg, 31, an elite Yelper from Portland, Ore.

Elites often rave about Yelp events online and then go back to review the businesses that put them on. Free food and drink may build up positive vibes and prime reviewers to view the providers more favorably when they return and pay out of their own pocket. But for their part, elites generally deny that the parties and freebies influence their opinions. “Businesses can't suck up to us,” says Mandana Yousefi, an elite Yelper. “We're critical people, and you can't buy our reviews.”

Yelp has also long been accused of showing favoritism to advertisers by emphasizing their search results and reviews. Some businesses say Yelp has tried to persuade them to buy ads by offering to make negative reviews disappear — or tried to strong-arm them by threatening to delete positive comments. In 2010, a few companies banded together to sue the firm over such practices. And some Yelp shareholders have sued, too, claiming the company inflated its revenues by coercing businesses to advertise. More than 2,000 complaints about Yelp have also piled up at the Federal Trade Commission, which has scrutinized Yelp's ad-sales tactics and “recommendation” software.

So far, Yelp has batted down all the claims. A federal judge dismissed the shareholder lawsuit last year, finding scant signs of “allegedly extortionate practices.” The FTC decided last year to close its inquiry without taking any action. For its part, Yelp says it takes “many steps to prevent gaming of our system and to protect consumers and business owners alike.”

Yelp's successes haven't stymied its critics, though. On sites such as Yelp-sucks.com, business owners and consumers continue to rant about the firm. An upcoming documentary, “Billion Dollar Bully,” takes aim at Yelp's tactics. On TV, Yelp's hefty influence in the ratings world has made it a satirical target. On an episode of the animated series “South Park” last year, elite (and insufferable) Yelpers tried to extract special treatment from restaurants, which exacted some unsavory revenge.