When I write an occasional happening, I want to engage the reader. To me, it's about getting other people interested in what you find interesting. So while I don't think a post should take up half a page, I do think it should be of a decent size. Three sentances tells me that the writer has no real passion for what they are posting, and why be the one to write it if you have no passion in the first place. I am for a few, short paragraphs and a pic or two, to best emphasize what you want others to see.
I'm also a massize narcissist, and I gives me a massive chubby when I write.