Way back in school, you were probably taught that contractions had no place in your essays and reports. School was to be for formal writing. You put two spaces after every period, you compiled a perfect bibliography, and so as not to sound like a kid you banished the contraction to the lowly realm of notes to pass to your friends while your teacher blathered on about spelling and grammar.
But this is now, people. You're not in school. Or maybe you are, but you're also online. And for all the varied things you write that aren't written to a teacher, please, for the love of gods, use contractions.
Don't know what a contraction is? It's when you combine two words into one, indicating the combination with an apostrophe. “You are” is contracted into “you're”. “Do not” becomes “don't”. “I am” becomes “I'm”. (If you have trouble deciding to use “it's” or “its”, remember that the one with the apostrophe is a contraction. Same goes for “you're” and “your”, and for “they're” “their” and “there”.)
Writers who don't use contractions sound like robots. So yeah, I'm gonna just go for broke and say it: CONTRACTIONS MAKE US HUMAN.
If you're at all concerned about connecting with your readers, give some thought to your use of contractions. If people tell you your writing doesn't sound like you, go see if you're using contractions.
And for the love of all that is holy in style and usage, when you comment on something online – which is perhaps the most informal of all types of online writing – use contractions. (Brother, this is for you! No more, “You are nuts,” on my Facebook wall, okay? Okay.)