It isn’t just that teens have phones, and that the infrastructure required to handle multiple simultaneous video streams is more accessible to developers than ever. It is also that teens aren’t getting out to socialize in real life like they once did. One in three teens told Pew that they hang out with friends outside of school less often than “every few days.”

“To me that’s where this story begins,” says Ted Livingston, founder and chief executive of Kik, a messaging app with 300 million registered users that is especially popular with American teenagers. It is hard for older generations to understand how young people cope with this lack of physical hangtime, he says. “The answer is they’re hanging out with their friends on their phone.”

Mr. Livingston says that when teens leave Kik’s group video chat open for hours, it is a sort of passive window into friends’ homes and lives. This behavior isn’t so different from the way Generation X would call friends after school, and millennials used AOL Instant Messenger and, later, text messaging to keep up with friends, says Ryan Hoover, founder of startup tracking service Product Hunt.

To a large extent, all these technologies have been an adaptation to teens’ inability to access one another in person, says Jan Odiaga, assistant professor at Rush University College of Nursing in Chicago, who studies how technology influences activity levels in young people. The situation is worse than ever because of packed schedules, helicopter parenting and the decline of walkable neighborhoods.