Whence the post in post office?
The term comes from positus, Latin for “position" or “station,” and a postal system carries information from one place to another, preferably with dispatch. (“Posthaste” first appeared as an instruction on the cover, or outside, of a letter but soon became a synonym for “hurry up!“)A blog post is a post of a different kind. The first Oxford English Dictionary citation for this kind of posting (“a message displayed on a mailing list, newsgroup, or other online forum to which it has been sent”) dates from 1981. The shorter post dates from 1982. The OED sees the influence of old-fashioned posting (“the dispatching of letters, etc., by a messenger riding post“) in these newer uses. In my mind, a different sense of posting has always been behind the online term: “the action of putting up a notice on a post, wall, etc., or of making anything public by this or similar means.” See also the Facebook metaphor of writing on someone’s wall.
Winifred Gallagher, How the Post Office Created America: A History (New York: Penguin, 2016).
I hope that this post makes someone curious enough to look up mail (as I just did).