I joined Facebook fifteen days after it launched, becoming the five-thousand-two-hundred-and-fifty-eighth user. I remember the early Facebook well. Back then, it was still called thefacebook.com, and you had to have a Harvard e-mail address to join. You could browse profiles. You could request friendships. You could “poke” people. But you couldn’t do much of anything else. At the time, Facebook event invitations hadn’t yet been invented. Still, students browsed profiles to determine whom they wanted at their dorm-room bashes. One evening, one of my old college roommates was invited to a party by someone she had never met; he’d liked her profile picture. Inspired by his picture, she decided to go. They are now married.

Today, Facebook celebrates its ten-year anniversary. To be honest, I didn’t think it would still exist. Apart from photographs and school affiliations—and the blue-and-white color scheme—few aspects of thefacebook.com have survived. My first wall post, from one of my closest friends, came on September 11, 2004—just over half a year after I’d joined—and was a joke about the nature of the wall. None of us were quite sure how to use this mysterious new white space in the middle of our profiles. Facebook hasn’t entirely abandoned the wall, but the concept has mostly been replaced by the news feed. The site long ago shed its Harvard-only roots. The feature that used to allow people to organize themselves into “groups,” which once seemed exciting and fear-inducing (Oh, please, ask me to join your group! Come on, you already asked my roommate!), was replaced with a newer, more news-feed-friendly version. I’m no longer a proud member of the Coalition for High-Heeled Women on Cobblestone, I’m sad to report. And although the poking feature still exists, it has been buried behind the site’s more useful tools. Despite all the changes, however, one thing has remained the same: the reason people join in the first place.

In 2012, the psychologist Robert Wilson and some of his colleagues reviewed the Facebook-related research that had emerged since the site launched to see if it might shed light on the patterns behind the network’s users and its expansion. While the reasons for joining and using Facebook were not entirely homogenous, one factor kept emerging as the strongest motivation for use: the desire to keep in touch with friends. Sure, some people joined because of social pressure or expediency, but the overwhelming majority of users were looking for something much more fundamental: social connection. Facebook had emerged as the best, most accessible, and most efficient way to maintain bonds with others.

There’s another, related factor, though: the desire to broadcast the nature of these bonds. “Apes groom each other as a way of maintaining connections and making those connections public,” Sam Gosling, a psychologist at the University of Texas at Austin, told me. “That’s what Facebook does. It’s a way of publicly grooming your friends. Those conversations that happen on people’s walls could just as easily have happened in private. Facebook allows us to meet this very basic social need, and to do that on a broad scale.”

In other words, it’s not just the connection itself that matters. It’s easy enough to support someone in private but far harder to voice that same support publicly—and the public support is a much stronger sign of actual support. (Remember the next-door neighbor who would play with you only after school and ignore you in the hallways?) If Facebook had stayed in its original incarnation, it might indeed have gone the way of other, forgotten sites like Friendster. But it went beyond that. The wall and its comments, all those “likes” and “shares,” and, yes, even the once-maligned News Feed are so powerful because they make our grooming of one another more public.

At its core, Facebook hasn’t changed what it does. It has just learned to do it more, and better. But, aside from the changes to Facebook’s features, there’s been another development: the number of Facebook users has multiplied. Not only are we affirming our connections in a way that sends a strong public signal, we are doing it with a lot of people at once. “We’re being allowed to essentially scale up and maintain our social networks and connections,” Gosling said. “That’s one of the reasons people become so obsessed with it—and freaked out by it.”

As Vauhini Vara pointed out the other week, Facebook is no longer growing as quickly as it used to. As the site reaches its inevitable saturation point and becomes less appealing to a younger generation, it will become ever more important to make certain that the site’s existing users don’t abandon it for newer—and, in some cases, simpler—ways of making connections, like Instagram or Snapchat.

At the University of Texas at Austin, Gosling and one of his graduate students, Gabriella Harari, have been examining why people decide to leave Facebook. They have found three broad themes: people see Facebook as pointless and unnecessary, they see it as a problematic distraction, and they are worried about privacy. As you experience a constant stream of updates from more people, the possibilities for distraction or frustration at a pointless update (did I really need to know that her baby is now teething?) rise apace. And as you share more information with more people, it all becomes a window into who you are—even the parts you might prefer to keep private. The more publicly we form and affirm social bonds—and the more people we form and affirm them with—the more likely we are to see our mental bandwidth filled and our privacy eroded.

Increasingly, many Facebook users are using the site the way I used to use it in college, deploying privacy controls to make their own Facebook more self-contained. A Pew poll last May showed that teen-agers—often expected to over-share—are now opting for private profiles more than sixty per cent of the time, and are often limiting what they share and whom they share it with. Fifty-nine per cent have deleted or edited a post, fifty-three per cent have deleted comments, forty-five per cent have removed tags from photos, and fifty-eight per cent have blocked a friend. All this socializing can start to wear on your nerves.

Photograph: The Asahi Shimbun/Getty. Chart data source: eMarketer.