When I took my first editing course, our instructor gave us a valuable piece of advice: don’t correct someone else’s grammar/spelling on social media. Ever.
Facebook, Instagram, Reddit, and internet forums (are they still A Thing? Am old) are the textual equivalent of a casual conversation. Do you stop someone mid-conversation to correct their grammar? No, you (mostly) do not. And you (mostly) don’t because that’s (mostly) a douche move.
Similarly, unless specifically asked, don’t correct someone on social media. It’s also a douche move. Use your superpowers for good.
I adhere to this policy…with one exception: If someone else corrects someone on social media and they happen to be wrong, then all bets are off. I say that, if someone else is abusing their superpowers (or perceived superpowers), feel free to jump in there and tear them a new grammatical/orthographic asshole. Back it up with quotes from dictionaries, style guides, grammatical textbooks, whatever you need. Make them regret the day they ever picked on the poor original-poster. Really give them hell.
And then (and this is key), explain that correcting spelling/grammar in a casual online forum is what assholes do. Tell them grammar teachers say so. Don’t be an asshole, pedantic Facebook-corrector. No one asked you.