When 16-year-old marly king, a high schooler in Birmingham, Ala., wanted to tell friends how excited she was about her brother's return from college, she could have sent a text. Two years ago, she definitely would have.
But now, she says, she doesn't "have a need" for that; she uses photo-messaging instead. "It's a more interactive way of communicating," says King, who sends 50 to 100 photos each day--including, at one point, a selfie with her sibling to alert friends about her brother's return--using Snapchat, the app that lets users exchange images (either plain or with virtual scribbles) that disappear after...