I get paid to review products but users to sign up get paid from google adsense. It's just a free easy way to make a little extra money by sharing your opinion. Like anytime you have a good or bad experience with a product or service, you can write about it. For an example,

http://www.ratinghq.com/4-12467/lady...-centers.bhtml

The google ad that is on the right hand side is my ad.