For a glimpse of the changing social media landscape for today’s teens, consider the Witt sisters of Champlin, Minn.: Courtney, 20, and Claire, 16.
They have Facebook pages but only check them a few times a week. Still, their online social lives are bustling—on sites like Snapchat, Instagram, and Twitter.
Why? "Our parents are on Facebook," Courtney said.
While adults are flocking to Facebook, teens are migrating to newer, cooler networks that Mom...