Although not as impressive as the graph of newspaper ad revenue illustrated earlier, the graph of first-class mail processed by USPS is still a fairly imposing cliff. Wall Street Journal has the graph in their article Postal Cuts Are Dead Letter in Congress.
Peak in about 2001, slow decline through about 2007 and rapid decline since. Volume is about the same as in ’85 or ’86.
Check out that graph and you will see why the USPS has revenue problems.