Do you feel the need to do so even when people are not making mistaked at all, but simply making stylistic choices which would have offended your English teacher back in the dark ages?
Then, frankly, you can fuck off back to your own Facebook/Journal/Blog/Whatever, and rant about The Youth Of Today _there_.l
Because that kind of behaviour gets in the way of proper discussion of what the person was talking about.* It turns it into a discussion of a piddling side-point which is almost certainly utterly irrelevant to the point they wished to discuss.
What it effectively does is say "When I was young, I had this set of rules beaten into me so much that I twitch whenever I see someone break them. And so I am now going to beat them into you." It's derailing, it's bullying, and it's the kind of behaviour up with with which I will not put**.
So, from this point onwards, I'm going to delete comments on this journal (and on FB, where I already made this announcement) which do this***.
(Exception: This post. Feel free to argue about it in the comments.)
*Unless, of course, said discussion is _about_ grammar, in which case feel free to let loose.
**Yes, I know that Churchill didn't say that
***To quote ciphergoth "There'll be a lot less comments like that by the time you've finished."
Original post on Dreamwidth - there are comments there.