I remember seeing this on Kottke.org, which may have been where you saw it, too, Tim.

Very little gets destroyed by new technology. Instead, its function changes.

When letters were the primary means of conveying information, including important/timely business information, service in the metropolis was faster and more frequent. Ditto for the newspaper, when it was the primary means of conveying news, opinion, popular entertainment, etc.

As different technologies, from the telegraph to the telephone to Twitter, have taken over these functions from email, the function of the post has changed. We no longer send short letters with the expectation of an immediate response. But we continue to send letters because they are 1) relatively cheap, 2) official, and 3) high-prestige. Letters' value in both technical and non-technical setting becomes tied to the signature -- which in early 19th-century England was largely a security measure (signature or signet across a wax seal = letter has not been tampered with), and only secondarily a voucher of authenticity as such.

This gets funny (from a historical perspective) with things like the newspaper, which had been the bang-smash, cheap-paper, fast-as-we-can-get-it disruptor of traditional reading, but which now is dressed in somber, grey-lady, "we are the last best hope of both democracy and traditional literary culture" garb. Even if you believe that this is the case, you have to admit that the newspaper's social function has fundamentally changed since its introduction -- and probably several times over. 

So too with hardbound books. Even if ebooks began to be the dominant way in which people consume literature -- I doubt it, because if paperbacks couldn't dust off hardcovers, I don't see any reason why ebooks necessarily will -- hardcovers will likely persist, but simply change in their cultural value and function. Just like the mail.

Concerning the time in which they are talking about when Austen was writing letters, the English Postal system had two great advantages.  England is fairly small, and there were far fewer letters being mailed.  Imagine a time when there was no junk mail to take up a sorter's time.  Look at the literacy rate at this period.  The reason you might have six deliveries in a day was the mail went out to the recipients as it came in on carriages from around the country.  London mail to somewhere else in London would arrive at your door in the morning, from Oxford later in the day while something from Ediburgh or Aberdeen would take considerably longer.  When the postal carrier only had a bag of letters to deliver he could afford to go back and pick up the most recent batch to arrive.

Here in the states, before the telegraph, we had much farther distances to travel.   The Pony Express was prohibitively expensive, around $5 a letter from east coast to west.  With growing literacy rates and cheaper postal rates, the mail became more popular.  When I was growing up in upstate New York, I can remember we had twice daily delivery.  This was in the 60s & 70s, and even at this time, there was no such thing as junk mail or direct marketing.

Now the post office handles I don't know how many pieces of mail a year (I would assume it's somewhere in the hundreds of millions) and it seems that most of the time, my mail goes from my mailbox to the paper recycling bin.

But let me ask you, especially at this time of year, are you more excited about recieving a Christmas card in the mail or an e-card in your in box? 

In London in the 19th century, there were vast numbers of poor who could be hired for virtually nothing to work very hard for very long hours.Plus, being a contained geographic area, London could centralize and do things very efficiently. Amazing delivery schedules were therefore possible and economical.

Outside London, the situation could not possibly have been the same. Mail from London to Hampshire would have been less frequent and slower. From London to Paris less frequent and slower still. From London to New York, MUCH slower and less sure of being delivered at all.

But people back then took communication by the written word very seriously. We take it for granted now. And I've run across articles saying that some schools are now considering not even teaching cursive writing because it is so pointless when people can just use a keyboard.

You gain something, you lose something.

Letter's in Austen's time were generally postpaid. That is, the recipient rather than the sender paid for their delivery. (Could you imagine if you had to pay your ISP or email provider a penny every time someone sent you piece of spam?) When you read nineteenth-century writers, some of them bitterly complain about the mails -- unwanted and unsolicited messages, poorly written gibberish, bothersome requests from family (which then you had to pay for). Reminds me of a lot of email I get now.

Prepaid postage coincided with the simple 1 cent per letter postage mark, often called the penny post, c. 1850. Initially the government sold envelopes with postage marked on the front; they fairly quickly moved to stamps when they realized how much letter-writers had already invested in envelopes. (The penny post was the original 99 cent mp3 or $9.99 ebook -- a cheap, memorable, and predictable price point that made up in volume what it lost in margin.)

The geographical advantages of England relative to the US aside, greater volume actually made the post MORE efficient, not less. (Imagine -- instead of a single letter going to Hampshire, you have dozens.) Ditto, the penny post corresponded with a tremendous literacy boom, which it helped advance by facilitating correspondence courses in reading, shorthand, teacher training, languages, etc.

Harold Innis has written at some length on how the post office's courtesy of free or reduced postage to newspapers, magazines, and other media helped foster a wider reading/media boom. In turn, the newspapers fought to keep postage rates low for generations. When that mutual interest was uncoupled, rates spiked.

Pre-telegraph, Charles Babbage cooked up a scheme whereby letters would be sent in buckets over great wires connected to church steeples, using pulleys and steam power to move them overland from town to town. An early imaginary vision of the electrical/electronic grid.

Likewise, the British postal service paved the way for government communication utilities like the BBC. Government monopolies over the post made government-run airwaves a natural. The US had a much more uneven development in this regard, a mix of private and public, sometimes both.

A terrific book on the history of the postal system (especially if you're into German literature and philosophy) is Bernhard Siegert's Relays. Google Books also has some great primary texts; too many to link here.

If you are having any sort of problem with a company (customer service, warrentee, &c), a written letter will get action must faster than a dozen emails.  Does the power of the pen still outweigh the electron?