A post is something you put up for others to see. The meaning does not imply that it is part of a larger context or a reply to anything. As such, a "post" typically is the start of a conversation or a stand-alone article. Often this is a blog post or something similar.
An entry is something you add to some larger context. You can add an entry to an ongoing journal, for example. The implication here is that what you've added is a single part of a larger collection.
A passage is a bit of prose, usually no more than a few paragraphs. It is often the bit of text covering a single idea excerpted from a larger essay. This word is most commonly used to describe text read or copied elsewhere rather than some original writing, as in "The teacher then read a passage from Don Quixote to illustrate his point." You rarely see this word used to describe something you'd post online.
A message is something sent to someone. The word implies that it has an intended recipient. In an online context, this is usually direct communication between two people, such as you'd see with SMS texting.