... correct someone's spelling or grammar on a Facebook or other forum post? I feel it is impolite to correct someone's spelling or grammar publicly, but have on a few occasions sent a private message if I feel that the correction is important. But most people would perhaps not notice or care about spelling or grammar issues if the meaning is clear.
I've just seen a Facebook post: "Thank you for excepting me into this group." It could be spell check at work against the poster (I'm sure we've all had that happen
). I usually put my red editing pen aside for this sort of thing because I know I often make mistakes myself, but sometimes I have to grit my teeth.