I think some people in this thread are confusing slang and general mistakes (typos) with people actually not knowing the difference. I have to admit that some do annoy me too, especially the 'bias' one, and the his/he's one, but I have been known to make the odd typo, and sometimes purposely misspell words because it's quicker, and even to use words that aren't "real" words.
For example, you will frequently see me type things like 'u' or 'ur', but that does not mean I don't know how to spell 'you' and 'your'. I might type no instead of know. I will also frequently not put apostrophes in words like don't and also not capitalise 'I'. I may even type color instead of colour. The point is, doing this does not make me stupid, and it does not mean that my school didn't teach me English properly. It just means that I put more effort into the point I am making than trying to appease the grammar cops.
One of my pet hates is when someone dismisses a perfectly good point just because they used their instead of there. That does not make you smart or better than anyone. You just come off as an ass that is too dumb to make a real rebuttal.
The fact is, English is a tool, it is used to communicate, as long as you can understand someone, then who the hell cares if they confuse uninterested with disinterested or spell definitely wrong. The beauty of English is that it is able to evolve, people can actually make a word up and because of context, people will know exactly what they mean, it is a good thing!
Now I'm sure you will all pick out the many grammar and spelling mistakes that I have most likely made and tell yourself that this means I'm an idiot and therefore everything I just said is idiotic.