A titleless post is not a micropost. By definition, a micropost is "a very short posted message." The hyponym is tweet or toot.

Microposts are generally short, but titleless posts need not be. They can ramble along for as long as the thought warrants. Calling a titleless post a micropost is like calling a subject-less email an SMS. Both lack a particular property but are inherently distinct.

Microblogging requires a different mindset, one that I have yet to master. However, microblogging is still blogging—it's in the name, after all. A short post is still a blog post.

The reverse, though, is not true -- with title or without. The title cannot be a mandatory field for a blog. However, a post without a title should not be considered any different from a regular blog post. Calling it "short" is entirely wrong.

Update: Microblogging platforms generally tend to have tiny little text boxes—a simple test for microposts.