The United States population just keeps growing and growing ... when you account for births outstripping deaths and add immigration on top of that, there's a net gain of one person in the U.S. every 17 seconds! Still, that growth tends to be heavily concentrated in the already-densest parts of the country. There are broad swaths of the country that are nearly empty, and just continues to get emptier. There are more than 3,000 counties (and independent cities) in the U.S., but in slightly more than half of all those counties, their population peaked some year in the past ... sometimes a decade or two ago, sometimes centuries ago.
To better illustrate that, I've put together a map that shows just when each county's population peaked, according to the decennial Census. The darker the color, the further back in time the population peaked. The yellow counties are as populous as ever; orange counties peaked in the later part of the 20th century; red counties peaked in the early part of the 20th century; and by the time you get into the purple counties, they peaked back in the 19th century. As you can see, it's not a random spray; instead, there are waves that steadily march across the country.

Below the fold, there's an interactive version of the map that you can pan and zoom and mouse over particular counties to see the specific year. There's also additional discussion of what the patterns that you're seeing might mean.