When researchers at Yahoo Labs wanted to learn more about how young people use video chat, they asked 16 teenagers in the greater San Francisco Bay Area to keep a diary for two weeks. The subjects liked to multitask, cleaning their rooms and scrolling through social media, while talking with friends. Chats sometimes lasted almost all night—teens would go down for dinner with their families, only to come back upstairs and resume the conversation.

“They would turn on a video chat and then just throw their phone some place,”...