Congratulations on 4 million users! The visualization works really well, IMO.
A couple thoughts on its implementation:
1) The dropdown list of countries is not alphabetized. Makes it difficult to find reviews from a given country, although it does make you use the map:)
2) If you click on a country that doesn't have a review, it produces a blank, instead of the country name and flag with no reviews.
2) If you click on a country that doesn't have a review, it produces a blank, instead of the country name and flag with no reviews.