Yep, its essentially the biggest flaw of Yelp. They can't figure out the fake reviews because its impossible to do.
I wonder how much they actually do to stop it though. How hard can it be to detect a user from Bangladesh reviewing a restaurant in San Francisco?
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Yep. I bet they got a ton of traffic because of this. However, I think they lost a lot of appeal to those who are looking for accurate representation of businesses and will most likely try and look elsewhere for that info.