Some people can't spell. Expecting people who don't spell well, and have no need to in their day to day lives, to use flawless grammar and spelling on a network that is for socialising, not spellchecking, is just prickish and pedantic.
I don't do maths in my head particularly well, and every time people act like everyone should have perfect grammar all the time and victimise people for it, I hear my family's voices when I don't add things up immediately in my head or don't know my times tables off by heart (I have to work them out, I never memorised them).
Different people have different skills.