"The main intention I had was to make positivity the path of least resistance," explains Justin Rosenstein, one of the four Facebook designers behind the button . "And I think it succeeded in its goals, but it also created large unintended negative side effects. In a way, it was too successful."

On February 9, 2009, Facebook introduced the like button. Initially, the button was an innocent thing. It had nothing to do with hijacking the social reward systems of a user's brain.

"The like button, simple as it was, tapped into a bottomless font of social feedback," explains Adam Alter, author of Irresistible: The Rise of Addictive Technology and the Business of Keeping Us Hooked . "And I don't think social media companies are trying to make 'addictive' platforms, per se. But since they're all competing for our (limited) time and attention, they've always been focused on making the most engaging experience possible."

But the problem is that attention isn't some non-sentient resource like wheat or oil. Attentiveness is a human state, and our reserves of attention are finite. They're hemmed in by sleep, work, children, and relationships with friends who find it rude when we're on our phones. So ideally we'd want to invest our limited supply of attention on things that make us happy. But as Facebook observed, social feedback induces a burst of happiness so brief it's addictive, causing us to return more and scroll further.

"The attention economy" is a relatively new term. It describes the supply and demand of a person's attention, which is the commodity traded on the internet. The business model is simple: The more attention a platform can pull, the more effective its advertising space becomes, allowing it to charge advertisers more.

Today, most of us reach for Snapchat, Instagram, Facebook, or Twitter with one vague thought in mind: Maybe someone liked my stuff. And it's this craving for validation, experienced by billions around the globe, that's currently pushing platform engagement in ways that in 2009 were unimaginable. But more than that, it's driving profits to levels that were previously impossible.

Following the introduction of Facebook's like button in 2009, YouTube moved to a binary like/dislike format in 2010. Instagram launched that same year and came ready-made with a like function shaped as a heart. Twitter adopted this same heart-shaped system in 2015, while, in the years since, Silicon Valley has come up with a multitude of new ways to gamify our need for social validation.

You know when you open Instagram or Twitter and it takes a few moments to load updates? That's no accident. Again, the expectation is part of what makes intermittent variable rewards so addictive. This is because, without that three-second delay, Instagram wouldn't feel variable. There's no sense of will I win? because you'd know instantly. So the delay isn't the app loading. It's the cogs spinning on the slot machine.

This is the most obvious way social feedback drives platform engagement, but others are harder to spot.

The easiest way to understand this term is by imagining a slot machine. You pull the lever to win a prize, which is an intermittent action linked to a variable reward. Variable meaning you might win, or you might not. In the same way, you refresh your Facebook updates to see if you've won. Or you swipe right on Tinder to see if you've won.

Former Google designer and ethicist Tristan Harris lays out the most common ways we're being manipulated on his blog. And as he explains, all of them use something called intermittent variable rewards.

Another piece of psychology hijacked by social platforms is that of social reciprocity; if someone pats your back, you'll feel pressure to pat his or hers. Facebook exploits this by alerting you when someone has read your message, which encourages the receivers to respond—because they know you know they've read it. And at the same time, it encourages you to check back to read the inevitable response.

The same bits of your brain get a rush on Facebook as a set of wavy dots appear as someone writes a message. You might not exit if you think you're getting a message, or at the very least you're more likely to come back. And while Apple also employs this feature, at least it allows you to turn it off.

All this might seem a little underhanded, but it's nothing compared to some of the design features currently showing up on Snapchat. Of these is the one causing the most concern, and uses elongating red lines to display the number days of since two users interacted. According to Adam Alter, this design feature is so effective that he's heard of teens asking friends to babysit their streaks while on vacation.

"It's clear here that the goal—keeping the streak alive—is more important than enjoying the platform as a social experience," he says. "This is a clear sign that engagement mechanisms are driving usage more than enjoyment."