Teen 1: Oh here comes that cannonball guy. He's cool.
Teen 2: Are you being sarcastic, dude?
Teen 1: (Hangs head) I don't even know anymore.
The episode aired in 1996, and many cultural observers could say that it sounded the death knell for the nineties' obsession with sarcasm. Mind you, the late nineties would continue to beat this dead horse for some time. But keen viewers could already identify with teen #2's question. By 1995, the widespread use of sarcasm had created a social sphere for youth in which genuine feeling couldn't be expressed without being met with sharp suspicion (or at least a confused grimace). Irony had established itself as the norm to such an extent that speaking genuinely and directly to a person (i.e. through a compliment) would require follow-up, explicit assurance that you were not being sarcastic.
By the time the new millennium rolled around, sarcasm was still a staple of pop culture. In the 2001 film Ghost World, for example, the main character Enid (played by Thora Birch) responds to a high school acquaintance's idea of hanging out some time with the line: "Yeah. That'll happen." The remark contains all the cutting irony of the mid 90s. But by 2001, the effect of this line had already become tinged with sadness. As the movie continues, we learn that Enid's sarcastic distance from the world of genuine expression alienates her from people and contributes to her malaise. She comes across as a female version of Holden Caulfield, endlessly criticizing the world for being full of phonies, then turning around and complaining of loneliness in the same breath.
So what does all of this have to do with the evolution of the exclamation point, you ask? Well it's pretty simple, really. Somewhere around 2005-2006, with the rise of instant messaging and mobile texting, the exclamation point became a form of punctuation that firmly said "I am not being sarcastic." Where ten years earlier, you might have met a comment like "Great job today" with suspicion, the inclusion of the exclamation point became a visual cue to accept the comment as genuine. In other words, the exclamation point could still convey enthusiasm or excitement; but this was no longer its primary function. Its primary function was to signify sincerity.
Today, a quick scan of Facebook profiles, Tweets, and mobile phone messages will reveal an explosion of exclamation points. The importance of this evolution in punctuation should not be underestimated. It does not mean that all young people have suddenly started taking Prozac (the last major rise in the use of antidepressants, ironically, came around 1993). Rather, it is a clear historical effort (maybe an unconscious one, in many cases) to distance oneself from the age of sarcasm-as-norm and to usher in a world of neo-sincerity.
It might seem strange to suggest that sincerity was something the mid-2000s needed to rediscover through the repurposing of a punctuation mark. But if you were to draw up a list of the ten biggest ways in which new communications technologies have influenced the emotions of today's youth, you would be wise to include the changing role of the exclamation point.