"All Internet humor is entirely reliant on you recognizing that thing you know, and nothing more." An article at Something Awful, the astute comedy site devoted to critiquing the Internet, explains why the Internet ruins all humor. Below, what would have happened if Seinfeld aired today. (In short, it would have annoyed you to death.)
If Seinfeld episodes like "The Soup Nazi" had aired ten years later, think of how much more grating the reaction would have been. Millions of images of Jason Alexander emblazoned with the caption "I LIEK SOUPS." Millions more pictures of the Soup Nazi, posted in retaliation: "NO MOAR SOUPS." Naruto music videos based on the joke. Soup references in every webcomic. Soup Nazi cosplay. Peak Oil reached. Ron Paul elected President. The world's volcanoes erupt in unison. All because the Internet ruins everything.
In real life, references live for a short time but only rarely invade the public consciousness; original humor has an advantage. But online, the cost/benefit ratio of quoting, "sampling" or "remixing" someone else's fad outweighs that of coming up with something on your own, so everyone just parrots catchphrases on their t-shirts and blogs and webcomics. Because the Internet lets normal people make as much noise as funny and original people, the lame humor that usually dead-ends in offices instead spreads like crazy.
Of course the same thing happens more and more in other media; MTV makes whole series revisiting a past decade or an old band. Is this just something that happens every generation, or are we really drowning original humor in a sea of catchphrases?
By the way, the article pokes at several other truths, like "The worst of [Internet humor] is 'random' humor" and "Because if you scream a joke as it's being told, it's like you're telling it yourself!"