I never do this with people that I do not know well. However, I have a young friend that is dear to me, and l call him my "heart-adopted" son. His spelling and grammar are atrocious, and I do correct him on it when I read and comment on some of his Facebook posts. He lives in another state now, so this is our main form of communication, sadly. The way I do it, is non-confrontational in nature. For example, he might post the following: "I worked rilly late last nite", and I would comment, "I'm really sorry you had to work so late last night". He knows that I am doing this to help him, and he is not offended by it.