If you're an independent business in the service industry, you already know that you live and die, to a significant extent, on online reviews. People who don't know who you are, which is a lot of people usually, go online and look you up before they decide where they're going to eat or shop or visit or book a hotel room. You get good reviews online, business bumps up- according to a Harvard study in 2011, a one-star bump on Yelp increases an independent restaurant's business by 5-9%. You get bad reviews online, it costs you a ton of money and sometimes drives you out of business entirely. Anyone who uses eBay knows full well what damage even 'neutral' feedback can do. So you want good reviews.
There are a couple ways to get good reviews. One is to be a good and reputable business people actually want to patronize. But that's the boring way and it requires work and stuff. Another way is to cheat your pants off by paying someone to leave you a good review, or even writing it yourself. A third way is to cheat someone else's pants off by paying someone to go to your competitors and leave bad reviews. But these are the dirty-rotten-scoundrel ways and require tricking customers and getting yelled at by Gordon Ramsay at some point and discrediting the legitimate reviews on the site for being mixed in with the fake stuff.
Some of those customers might be New York Attorney General Eric Schneiderman, who today announced an investigation that had been going on for the past year called Operation Clean Turf. What happened was, investigators ran a good old-fashioned sting operation. They made a Yelp page for a yogurt shop that did not actually exist, and then went around to suspected fake reviewers and offered money for good reviews. They got them, from places like Bangladesh, the Philippines and eastern Europe, for amounts ranging from $1-10 per review.
They caught 19 different companies.
Now, there are other issues with online reviews (this is not to say the reviews are entirely untrustworthy, so don't get that impression). Sometimes people review places that have yet to open (you see this with all kinds of things that can be reviewed online, movies, video games, the lot). Sometimes people hold good reviews hostage in exchange for the restaurant giving them free stuff. Sometimes people misrepresent things that were their fault as the restaurant's fault, be it accidentally or with actual malice. But this is a part of the problem you can more effectively fight.