These are people, usually misinformed, who go about correcting everything, even when they are wrong.
They become especially militant when it comes to social media and email, refusing to acknowledge things like autocorrect, which can make writing on the fly hazardous. In the age of texting, we should be forgiving: texting is an informal thing, after all. It has more in common with note writing than with letter writing. And immediacy is the whole point.
I’ve met people who consider themselves “grammarians” (the word they use). They think that what they learned in grade school still applies, when actually, it often doesn’t. The result is, more often than not, confusion. A true grammarian looks for documentation. Ask these self-proclaimed experts the last time they opened a dictionary. If they haven’t done so within the last week, they probably don’t know what they are talking about.
Good grammar is not a moral issue, although many people think it so. Sometimes, it’s time to lighten up, rather than go to war over unimportant things.