certainly the experience is very different if you live in, say, New York City, than if you live in Istanbul and sometimes travel to New York City.

Though the 'cool kids' aren't using it, Yelp has basically cloned the 4sq product within it's mobile app (baron instead of mayor...). They come from a very strong position already having all that review content to support it. They seem like more a threat to 4sq than fb is.