“In 1790 the postal system transmitted just under 300,000 letters, or roughly one letter per year for every 10 inhabitants of the United States, excluding Indians and slaves. … By 1856 the comparable total had increased to around 130 million, or 5.3 letters per free person per year.
“No less notable was the regional variation. In 1856, in North Carolina, the average inhabitant sent 1.5 letters per year; in Indiana, 3; in Louisiana, 5.5; in Massachusetts, 10…. On average, urban Americans wrote between 5 and 20 times as many letters as rural Americans….”
— From “Spreading the News: The American Postal System from Franklin to Morse” by Richard R. John (2009)