If you spend enough time with the map, in fact, you'll see that old newspaper exchanges worked much like modern social platforms do. Exchanges were, before the invention of the telegraph, the way local community newspapers got their news: Editors at different papers would mail copies to each other, forming networks of different-sized papers with similar political views. Frequently, editors would even copy, verbatim, the stories that appeared in the papers of their fellow partisans. (Think of that as a retweet ... circa 1847.)
In other words, each of those newspapers formed a node in an informational network; the postal route between them was the Twitter follow or Facebook friendship; and aside from the obvious facts that these networks moved slowly and connected institutions (as opposed to people), they basically operated a lot like social networks on the internet.
If you type a term into the News Map, for instance, you can see that the most influential nodes tend to be the most central - not necessarily the largest or most popular papers, but the ones that are geographically closest to lots of others.
And you can see that, even when a lot of nodes exist in a cluster, they won't necessarily light up at the same time: Even when they're exposed to new information, nodes only pass it on under certain circumstances. Saunt points out that Democratic and Republican exchanges tended to share things only among themselves, for instance, regardless of where the individual papers were located.
Best of all, the News Map captures at least one viral hoax, as if to reassure us that humanity has always been this gullible. In 1864, an anonymous writer distributed pamphlets in New York, advocating for more interracial relationships and coining the term "miscegenation" to describe them. The word pops up in a local Democratic paper. Soon it's in other Democratic papers, spreading westward. Then counterpoints start cropping up in Republican papers.
Only when "miscegenation" had been printed in dozens of papers across the country did anyone realize the original pamphlet had been written by "satirists" to troll their political competitors. One-hundred-and-fifty years have since passed ... but the scenario sounds oddly familiar.
To quote the Economist's 2011 observations on Martin Luther's 16th-century (!) virality: "Modern society tends to regard itself as somehow better than previous ones, and technological advance reinforces that sense of superiority. But history teaches us that there is nothing new under the sun."
Try busting that out the next time someone bemoans the state of social media.