Is there not a forum in the world where this topic does not come up? My attitude to this is:
1) Is English their first language? If not, then I try to make sense of what they are saying and inwardly applaud them for trying to communicate in a foreign tongue. How many of us can speak a second language fluently and write in it without any grammatical flaws? Not many here I would hazard to guess. If yes then see 2a and 2b.
2a) Is the forum a technical forum? If it's a technical forum then I would want that person to be accurate, both in their spelling and grammar. Not only because they maybe giving detailed instructions for me to follow, but I'm sad to say and I admit to feeling this myself, it imparts a sense that the author is educated, knowledgeable and that they can be trusted in their advice. Of course this is not always the truth or any guarantee.
2b) If it's a social forum then MEH! lolcatz omfgwtflolbbq just relax. Many people nowadays write in much the same style as they speak and even the most educated of people speak in a much looser grammatical style than they may write. Indeed speech writers for major figures will write their client's speech in the style of their spoken word rather than how that person may write the written language so that the delivery will feel more natural and give the impression that the orator may actually have written it themselves.
So if you find someone's written text grating at your grammatical sensibilities then relax in the knowledge that you are getting a small insight in to how that person communicates vocally to people in the real world and deduce from that what you will and be content with that for it is unlikely that you will ever change something so ingrained in them as to how they have learned to speak. And for those who wish to continue writing in such a manner, well you can work out the obverse for yourself. You are all still intelligent people for the most part.