A complicated topic! Among Americans it seems like it partly comes down to age. As an over 40 guy I don't make much distinction between writing a letter, a text or a forum post. I attempt to use "white-space" (a term that didn't enter the lexicon for our average person until the late '90's) and correct grammar. But since a forum is often a free-wheeling, off-the-cuff environment I don't try to be a Nazi about it. And I'll use a few abbreviations, since this isn't formal correspondance.

The only real "capital crime" against readability is ALL CAPS, theendlessrunonsentence, and the steadfast refusal to use paragraphs.