Mail use has been changing over the past decade as businesses and consumers have moved to electronic communication and payment alternatives. Mail volume decline has accelerated with the recession, particularly among major users in the advertising, financial, and housing sectors. Mail volume has typically returned after recessions (see figure), but USPS’s 5-year forecast suggests that much of the recent volume decline will not return.
The GAO government site released a pdf format report with some startling graphs. The post office is in the same boat as newspaper publishers thanks to what is being called "new technologies", better known as email, cell phones and instant messaging.
The Post Office is vital to so many industries that I don't see it going away anytime soon. People still like print publications, etc., well enough that many of the larger publishers (I mean the ones who meet with postal committees to help determine pricing, etc.) will be willing to pay extra to keep the USPS afloat if necessary.
I don't see postal services going away completely either.
It did always surprise me to see people delivering mail on Saturdays in the US, that is definately a luxury that needs to go. Clearly people can wait an extra day for their credit card bill or publishers clearinghouse offer.