While I don't doubt volumes have fallen as a result of things like email, the US postal service also hasn't done itself any favors. You read anecdotes like this, and it's a wonder that volumes haven't plummeted even faster: http://www.theatlantic.com/business/archive/2010/04/things-a...
As a rule of thumb, if your gut tells you that a particular service is essential to a civilized society the service belongs in the public sector. As a nation we've decided the ability to affordability correspond with others around the country is such a service. Education is another. These two examples are extraordinarily complex problems which the US public sector solves well, all things considered. Yet every time we discuss these services someone always wants to privatize them.
For the most fundamental components of our society, privatization is never the answer.