One FB friend of mine constantly mixes up "his" and "he's".
So - "his really cute", "his starting school" but "he's friend" and "he's mum". So today I read "his going to get he's hair cut"!!
Another FB friend doesn't seem to understand the word "as" and writes it as "has". "has good has gold" etc!
Many others are guilty of defiantly/definatly, being "exited" about things, posting about having "brought" a new top, and an otherwise very good friend always writes "argh" when she means "ahhh" (if that makes sense? I'd imagine argh to express fright or disgust, when she means "ahhh the baby is so cute!")
I have to bite my tongue and sit on my hands every single time, because I'm pretty sure if I pulled them all up on it, I'd have no friends left!!!