I'm going to show both my age and my geekiness here.
The word "post" originally came from the function of "sending" or "posting" material. This could be to a BBS, to Usenet (a kind of distributed BBS system), to a blog, to a forum, etc. So it was originally a verb, i.e., "to post". From that came the descriptor "poster" (meaning "s/he who posted") and the term "post" in noun form, i.e., "that which was posted". So, really, a "post" is anything that is "posted", i.e., sent, published, shared, etc. On a less geeky level, I don't think there's much difference between an "article" and a "post". They're pretty much interchangeable at this point in internet linguistics and popular culture.