Dude has really crept like moss in the past decade or so. As with most slang, it doesn't ask your opinion before it comes knocking, and it doesn't care what you think of it.
What I'm trying to say is, males in America will hear themselves called dude, whether by friends, acquaintances, or strangers (it's common even in business transactions, if you are both, say, 12-40). However, it will always have an intimate, jocular, and (I can't back down on this) strutting air. "I think I'm pretty cool, and I just might let you in my club"-- this must form some part of the dude-sayer's mantra. Sometimes it comes across as innocent and harmless. Sometimes it's terribly obnoxious, really because it's so artificial. Mostly, the effect of dude is to quietly point to one's hidden army: thousands of other dudes, just like him, ready to back him up and affirm how perfectly cool and acceptable he is.
I would say that that is the irking menace of dude in the online setting. If you are pretending to be at all serious, the only reason to call someone dude in a forum is to appeal for help to the Perfectly Stupid Code for Dudes, Nerd Edition, which will say, plain as day, that you are right and that he is--: but of course. Not cool.