Harvard Study on Fake Yelp Reviews

It seems that Yelp discards about 25% of submitted reviews:
Yelp confirms Harvard study about fraudulent reviews

Some of the interesting points:

• Roughly 16 percent of restaurant reviews on Yelp are identified as fraudulent, and tend to be more extreme (favorable or unfavorable) than other reviews.
• A restaurant is more likely to commit review fraud when its reputation is weak, i.e., when it has few reviews, or it has recently received bad reviews.
• Chain restaurants – which benefit less from Yelp – are also less likely to commit review fraud.
• When restaurants face increased competition, they become more likely to leave unfavorable reviews for competitors.

I haven't read the whole study yet, but it's online here:
http://people.hbs.edu/mluca/Papers%20on%20RIS/FakeItTillYouMakeIt.pdf