Craigslist organizes posts into local, geographically oriented, lists.
The white areas out west are sparsely inhabited areas. This is most interesting for regions in the east.
What I think is kind of cool, is to pretend your taking a long road trip. Then following the Craigslist zones is similar to keeping track of the signs on the interstate, that are always showing you the city that is 2 hours down the road.
For example, I did Tuscaloosa to Buffalo, via Knoxville a bunch of times. According to Craigslist this is Tuscaloosa, Birmingham, Gadsden, Northwest Georgia, Chattanooga, Knoxville, Lexington, Athens (?), Cincinnati, Dayton, Columbus, Mansfield, Cleveland, Ashtabula, Erie, Chautauqua and Buffalo. If you put someone on the interstate, and told them to follow these signs, they’d have a good chance of making it there.
Here’s a variety of maps in different sizes.