Emojis are deliberate cues to the intention of what we're saying
The facial expressions are by far the most popular, and yet there's an important way in which they're not like our ordinary kinds of facial expressions. When we're interacting with other people, we find the most trustworthy kind of facial expression to be the kind that's given off involuntarily: the burst of laughter or sob in the throat that's difficult to fake. And yet you can't involuntarily give off an emoji. They're all given out deliberately—you choose exactly which one to send, and you know that everyone else does, too.
Emoji and all of their relatives are fake by definition.
If we try to say that they map directly onto our emotional facial expressions, then we have a weird mismatch. How is it that we're so keen on such disingenuous symbols? What's to enjoy about a world where everyone is wearing a mask?
A paper by linguists Eli Dresner and Susan Herring has a compelling answer. Rather than think about emoticons as emotional, they argue, we should think about them as deliberate cues to the intention of what we're saying. Sometimes that intention does align with an emotion: if you say "I got the job :)" you're indicating that you're happy about it. But sometimes you put on a facial expression aspirationally, the way you might put on a polite social smile during a customer service interaction, even if you're having a terrible day, just to make things proceed smoother. A smiley face might be used in a context like "I'm looking for some suggestions :)"—you might be anxious rather than happy about requesting feedback, but you're using the smiley to make the request more polite. Moreover, people sometimes use smiley faces in contexts that aren't happy at all. Dresner and Herring quote a person saying "I feel sick and tired all the time :)"—the speaker isn't happy or even smiling about feeling sick and tired, but might include the smiley to indicate that they don't want their words to be read as a complaint. The same statement with :(, on the other hand, could be intended as a request for sympathy.
When I design online ads for American Apparel, I almost always look for an angle that will provoke. Outrage, self-righteousness, and titillation all work equally well. Naturally, the sexy ones are probably those you remember most, but the formula worked for all types of images. Photos of kids dressed up like adults, dogs wearing clothes, ad copy that didn’t make any sense—all high-valence, viral images. If I could generate a reaction, I could propel the ad from being something I had to pay for people to see (by buying ad inventory) to something people would gladly post on the front page of their highly trafficked websites.