As I prepared the slides for a talk I gave yesterday in San Francisco, I started to think about the history of messaging. I came up with the following timeline (the early dates come from Joel Mokyr's book The Lever of Riches)...
In 1800, it took 2 years to send a message from London to Calcutta. You wrote a physical letter and entrusted it to a wind-powered ship that sailed down the western coasts of Europe and Africa, around the Cape of Good Hope, back up the eastern coast of Africa, across the Arabian Sea, etc. -- with, presumably, stops in just about every port (yes, they had multi-hop message transports back then).
By 1914, it took 1 month to send a message from London to Calcutta. The Suez Canal had opened, and steamships powered their way through the Mediterranean to the Red Sea, and thence to India. Big improvement.
With the advent of reliable airmail (1950s or 1960s?), the time was probably reduced to 1 week.
Overnight mail, which became popular and relatively affordable in the 1980s, cut that down to two days. (Remember those early FedEx commercials? "When it absolutely, positively has to be there overnight!")
When the Internet was opened to commercial use, electronic mail became the killer app (1994 or so), and people grew accustomed to delivery times on the order of 10 minutes (depending on the number of hops, how frequently you polled your mail server, and so on).
Then along came instant messaging. Now your message gets from London to Calcutta in something like 100 milliseconds (and almost always less than a second). Plus you've got presence information, too!
Does this mean faster is better? No. But when was the last time you sat down to write a physical letter and sent it via surface mail? Sure, you might do that once in a long while, but it's rare enough that you remember each occasion.
The lesson I draw is that people feel the need for speed. All other things being equal, people will prefer the fastest means of communication available. That doesn't bode well for the email network. But, as Doc Searls recently noted, email is a slum.