The site has created an entirely new category of relationship, one that simply couldn’t have existed for most of human history—the vestigial friendship. It’s the one you’ve evolved out of, the one that would normally have faded out of your life, but which, thanks to Facebook, is instead still hanging around. Having access to this diffuse network of people you once knew can be pleasant—a curio cabinet of memories—or annoying, if those good memories get spoiled by an old friend’s new posts, or helpful, if you need to poll a large group for information. But it is, above all, new and unusual.

It’s reasonable to assume that those with older Facebook accounts are likely to have more of these lingering vestigial friendships, or, as researchers often call them, “weak ties.” They’ve had more time to “friend” people and then fall out of touch. But even those who’ve joined the site more recently may experience this phenomenon. For instance, Rebecca Adams, a sociologist with the gerontology program at the University of North Carolina at Greensboro, just got a personal Facebook account for the first time last year. (She had a professional Facebook account already.) And she optimized her account so she could find some of these estranged connections: “I used my high-school picture when I got on Facebook,” she says. “I already knew which people would recognize me now, but what I was interested in was finding the people who knew me when I was young.” There’s a sort of shared understanding that connecting with people you’ve lost touch with is, at least in part, what Facebook is for.