For purpose of this answer, I am considering (a) Websites with user-generated reviews (Think Yelp, IMDb) (b) Websites expert/critic reviews (Think Zagat, Consumer reports) (c) Independent review blogs (d) Websites for which reviews are not the primary business (Think Facebook, Google). However, I have excluded businesses like Amazon and Expedia, which include reviews for products and businesses that they are selling.
Here are some popular monetization models for these sites.
- Advertising: This is the most popular form of monetization for review and rating websites. Usually, the "reviewed" side advertises to review-readers/writers, like restaurants advertising on Yelp, lawyers advertising on Avvo or movie studios advertising on IMdB. However, in some cases, businesses other than the ones from reviewed side of the platform may also target review-readers. For example: MTV shows their own ads on ratemyprofessors.com (You don't expect professors to advertise to their students, do you? :)). Independent review bloggers also use advertising, usually through content ads through Google AdSense or similar means.
- Subscription services to customers: Some websites may charge a subscription fee for access to their content. Examples include: consumerreports.org and Angie's List. With so much free content available on the internet, this model usually works only if the content is either premium (written by experts) or scarce.
- Affiliate marketing: Some review websites get paid by reviewed business on a CPA (cost per action basis). Examples of these include websites like nerdwallet and creditkarma, which get paid by the credit card companies. Many independent review bloggers also use this model. There seems to be some conflict of interest here, but most reputed websites manage to draw a good line between the advertising side and review side of their business.
- Licensing review data: User-reviews are very important criteria for consumers to make purchase decisions, and hence many businesses may be willing to pay for using review data from these platforms. Potential customers for this model include reviewed businesses (Example: Hospitals paying healthgrades.com to use ratings in their ads) and other advertising platforms (Example: Google licensing product rating data from TrustPilot, Bizrate and StellaService among others.)
- Selling premium account services to businesses: Another possible source of monetization is selling premium account services to businesses. The services may include enhanced listings, ability to include curated content (instead of just user-generated content), and opportunity to connect to reviewers among others. IMdB Pro and Yelp Business account come to mind as some examples of these.
(Last two are relatively less common monetization methods)
- Sale of review guides: With the decline of print media in general, this is not a very popular monetization method. Similar to (2), for this review model to succeed, the content needs to be premium. Zagat used to be a premium player in this field, but lately, they have gone primarily digital. Zomato in India also publishes print guides.
- Indirect monetization by keeping user on your properties: For businesses like Google+ and Facebook, the purpose of reviews is not really to monetize the reviews page directly. However, these review pages keep the users on their properties longer, and eventually, this may result in few additional ad-clicks.