I'm no grammar expert, as I'm sure many of my posts have proven. I'm also very prone to typos; I proofread, but just don't see a lot of them (my worst is interchanging "the" and "that" and "than"). And my Internet-speak is a lot less careful than my paper-for-school-speak. For a layman, though, I think I do pretty well. So when I pick up a newspaper or magazine or some other publication in which the grammar and spelling errors should be few and far between, and they're not, it drives me batty. If I could write and edit better, and I have no English degree or experience editing, why is this a job that requires a college degree?
However, in informal writing, such as here on eHell, who cares? I just don't understand. Some things bother me, yes, but not enough to say anything (and that's why I like the linguistic pet peeves thread; I can talk about it there if I must say something). Some posts I just can't understand, but still, I don't care enough to embarrass someone I don't even know.
In everyday speech, it's kind of understood that people will speak more casually. I often say something like, "There is 25 people in that line!" because I haven't thought the sentence out before I use "is" instead of "are," or I'm more prone to using pronouns incorrectly (me/I, she/her, etc). Some people who don't blink an eye at this kind of poor grammar are all over you if you do it on Facebook, which, IMO, is the written equivalent of everyday speech. I don't get it.
The ONLY time I pointed something out was when a friend posted a status update saying something like, "I hate all those idiot's who can't get their grammar right!" I asked her if she wrote "idiot's" on purpose, and it turns out she did; it was a joke in response to someone ELSE using poor grammar to point a typo in one of her wall posts.