Well for those of you who think grammar mistakes don't matter, how well a person composes their thoughts into words has more to do with beliefs (true or not) about that persons intelligence. That is even more true when the only way you communicate with a person is through text. When it's a conversation in person you won't notice the minor mistakes because you are looking at body language, thinking about your response, don't see the way things are spelled, and if its a girl looking at her figure (It's ATOT after all).
Let me pose this question to you. Say we have two identical people (say twins separated at birth); one has outstanding grammar skills and speaks English very fluently, and the other speaks in slang and uses poor grammar. Which person do you believe is more intelligent? Command of your language is very important in how intelligent a person you are perceived to be.
One or two mistakes is understandable, posts that a 2nd grader could compose better are just annoying.