If Facebook delivered every single piece of content to every single recipient, the way the USPS does, then maybe the analogy would hold. But they don't. They use algorithms to determine what to show and what to hide, and IMO they are responsible for that.
Imagine if the U.S. Postal Service could keep track of which pieces of mail get opened or read, and then they decided to only deliver the mail that made people the most angry. Sound like a good postal service?
That's because people can ship dangerous things in the mail. The post office isn't controlling information sent in the mail.
There's a big difference between opening something up (many times needing a warrant, as your link says) because it may contain a weapon or biological agent or invasive seeds of a plant species, vs controlling what information is passed or not.
Imagine if the U.S. Postal Service could keep track of which pieces of mail get opened or read, and then they decided to only deliver the mail that made people the most angry. Sound like a good postal service?