We've all done it, at one point or another. And if you say you haven't done it, you're either lying or will do it eventually. It is the way of the Internet.
"Um". Two letters, one syllable, so much attitude.
It positively oozes condescension, making an otherwise reasonable statement at least ten times more patronising. It's a handy tool, allowing you to express your utter disbelief with someone else's words or correct them in a manner that drives home just how wrong they are. It's kind of sneaky too because as rude as its use can be, only the strictest administrators and moderators would be inclined to police it. Again, two letters and one syllable, yet it allows us to express so much, sneer with so much attitude and pomposity, but 99% of the time it's not going to violate any rules or code of conduct. Um: how to say you're a fucking moron, without saying you're a fucking moron.
"Um" is kind of unique in this way when it comes to online banter. It can be used innocently enough, but more often than not when it begins a sentence, you know exactly the tone the poster is going for. "Uh" can be used in the same way, but it just doesn't feel as strong. "Er" can be used too, but it feels a little more polite and can just as easily be used to represent genuine confusion in the tone of a reply. "Eh" is pretty much interchangeable with "Meh" and displays ambivalence or lack of enthusiasm more than it does condescension. "*shrug*", "Pfft" and "Bah!" can be effective, but they too just don't have the same punch as "Um".
When it comes down to it, it's a great way of looking like a pretentious, dickish know-it-all. Assuming that's not simply part of our inner nature, what do we gratuitously use the word "Um", even if we despite know-it-alls and that kind of attitude in general?
Simply put, it's satisfying. Sure, when somebody posts something that we completely disagree with or is completely off the mark, we could say "I respectfully disagree" or "Sorry, but you're incorrect" before continuing with our argument or a correction. But when the other person has posted something so wrong and been so sure they're right, it's far too tempting to begin your reply with something like "Um, no." Enough attitude to put the other person in their place, not quite enough attitude to get banned in most places. Flipping the bird without the middle finger and fighting fire with fire; proving a know-it-all wrong by acting like an even bigger know-it-all. Just make sure what you have to say carries some weight.
We should be stronger than that, though. It's like sprinkling Internet shorthand into a discussion, arguing not with a counterpoint but a mocking tone to put the other person on the defensive ("LOL you really think that? SMH LOL"). Like twisting the other person's words and putting words in their mouth, it's a debate technique that's been taken to new heights (or lows, depending on your point of view) online. We should be stronger, but as a tactic it's too effective and in lieu of everyone adhering to some form of Queensberry rules, you either have to cut through it or resort to it.
It has its place though, so I'd never advocate banning it completely from the online lexicon. Rather, I'd propose self-moderation. Before you "um", think to yourself...just how wrong is this other person? How sure am I that I am 100% correct? Do I really need to be that dickish about a simple misunderstanding or obvious typo?
After all, there's nothing more humbling than having your "um" countered with an even stronger "um".