Half Of The US Population Lives In Less Than 5 Percent Of US Counties

From Business Insider, "Half Of The United States Lives In These Counties" by Walter Hickey and Joe Weisenthal:

Using Census data, we've figured out that half of the United States population is clustered in just the 146 biggest counties out of over 3000.

Here's the map, with said counties shaded in. [The original website has a list of the 146 counties shaded as blue.]

Source: Business Insider