I was one of "those people". Do you know the dipstick always correcting other people's spelling and punctuation online? That was me. I thought (for some) I was doing them a favor, one they just didn't ask me to do. For other people, I used it as a way to undermine their trolling.
I will still (at times) "edit for" trolls, especially when they're trying to sound intellectually superior to someone else. Truly smart people don't flaunt their intelligence to make others feel inferior (and most people who try such a thing tend to have atrocious spelling).
However, I won't give unsolicited editing to acquaintances. It's a crappy thing to do to someone who is just sharing random thoughts with the world. I even considered my behavior trolling, once I looked at it closely. A person on Facebook might not have English as a first (or even third) language. A disabled person might not have the dexterity to type flawlessly. Creative spelling and punctuation could be a conscious choice. Mistakes can happen to anyone.
There is a time for editing words. At the wrong time, it becomes annoying and hurtful. I'm thankful I realized it years ago... I'd be insufferable on Twitter!
Have you ever corrected someone's grammar or spelling on social media? If so, why?