Mass Media, Day One: The Post Office
Quick, name the federal agency you most recently had contact with. For a lot of people, the answer would be the Postal Service.
Its mission and importance have changed in the age of email, but it certainly can be argued that the postal agency is of vital importance.
In fact, that IS the argument of Winifred Gallagher's book How The Post Office Created America.
Gallagher points out that the Continental Congress created a postal agency before it did almost anything else. It predates the Declaration of Independence by a year. The author visits the Exchange with the determination of the Founders and the execution of their idea.