And you know what? I'm only partially joking. Catching grammatical errors and offering corrections is a necessary and underappreciated task ... when you're reading a book, a master's thesis, or a comedy article somebody wrote while drunk at an Applebee's.
But the problem isn't the action; it's the audience. If you're a Facebook corrector, then I hate to break this to you, but not one person in history has taken your *their* reply as anything but a passive-aggressive insult. The victims of your rampant grammar sprees don't find themselves about to type "Garys vacuum so shitty it cant even suck at street fighter" and then, recalling your past corrections, suddenly backspace to fix the error while giving a giant Mentos-style wink and thumbs-up to a nonexistent camera.
So if they're not going to learn a lesson from it, and you -- like all trolls -- insist that you're not trolling, then what's the point of correcting them? It's best to just learn to ignore these infuriating errors, and let all of human language slowly devolve into anarchic ignorance and pictographs. Because that's what is going to happen.
I'm sorry, it turns out you were the last gatekeeper of literacy, after all. Who could have known? Ah well, back to dirt drawings and marking ownership with urine.