Over the last few decades, it seems that schools haven't been focusing much on spelling. That might be one reason that children mix up "then" and "than" and just guessing what is should be, from the sound they hear. You see more spelling mistakes everywhere - over here, too. If smaller children keep reading texts with so many mistakes, they'll probably think it's right and copy it. I don't believe that bad spelling is solely because of texting so much, although it might be part of it.

Using "y'll" instead of everyone or everybody, I find quite charming/positive, because to me it gives a feeling of "togetherness," or good vibes from the person saying it, but I could be totally wrong, I'm not American.