Shorthand is great for instant messages, where brevity helps the flow of a live exchange. Usually when I’m instant messaging with someone, I’ll use all lower case and many acronyms (brb, g2g, cya, lol, etc.). But a discussion in a forum is an asynchronous exchange; there is a delay of seconds, minutes or even days before a reply is received. So brevity doesn’t help “keep the dialog moving.” In fact, it often has the opposite effect, instead requiring the reader to translate the slang into English (or whatever), ultimately slowing down the communication process.
Basically, all my posts in any forum generally use proper spelling, grammar, punctuation and sentence structure. Forums are more formal and structured than a chat room or an instant message. Unlike chat rooms and IMs, a forum discussion is read and re-read thousands of times. This permanence warrants a little extra time and care when writing.