People seem to treat forums, instant messaging, emails, etc like they do everyday conversation, so they simply type how they talk (including how it sounds) and pave over the distinctions that arise in written language (ex: it's and its are spoken the same way, so people just use one way of spelling for both. They simply aren't paying attention). Also, while we're on the subject, the reduction is much worse for you're/your, it quickly becomes "ur" in most internet speech. It seems that people are satisfied being able to talk, while fluency with the written word is simply second rate.
Knowing these distinctions will also effect people's fluency with the language in terms of speaking it properly. That's not to say that people won't find other ways to communicate using their lazy ass sort of language. If you actually pay attention to what you're saying (thinking before you speak), a good training in grammar will allow you to say what you intend more easily. It would be especially nice, however, if people respected grammar in the written word, but no one gives a shit. As long as it gets the job done right? Who cares if I'm crippling the breadth of what I can say, there's only a few things I need to say anyway because I'm a fuckin moron!