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Tripadvisor is much better in Europe and yelp is better than Tripadvisor in the US in my limited experience.

+1 this - I have been to London recently and Tripadvisor has much more reviews about restaurants/POI than Yelp.

Foursquare is worth checking out (over Yelp or complementary to Yelp) in a lot of US cities too, definitely in SF and NYC.