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Yelp's data would seem to indicate that users with a positive experience are more likely to write a review: http://www.yelp.com/press

I wonder if those figures are massaged, considering everything else going on!

That graph has been on the page since before any class action lawsuits were filed.

But that does not necessarily mean it is legit. Until the class action did we not think Yelp was totally legit?

Amazon has observed exactly the same behavior. The average review for something on the internet is 4.5 stars.

What about astroturfing by product vendors?

I am not sure what you mean by that. To me it seems something like: "well, most of the time, lynch mobs do catch the bad guys."