FB COO sees more teens on Tumblr, Twitter

Facebook , owner of the world's most popular social network, is facing increased competition for younger users from services run by Twitter and Tumblr, according to Chief Operating Officer Sheryl Sandberg.

While more teens are turning to rival social networks , Facebook usage is still growing among that demographic , Sandberg said on Wednesday in an onstage interview at the D: All Things Digital conference in Rancho Palos Verdes, California . "Teenagers are using other things more," Sandberg said. "At the same time, they continue to be very active, engaged Facebook users."

Attracting and keeping young members is a crucial part of Facebook's efforts to bolster advertising revenue and find ways to profit from the growing portion of its more than 1 billion users who access the service on mobile devices. While about 94 percent of teen socialmedia users had Facebook profiles last year, their ranks on Twitter more than doubled to 26 percent from 2011, according to a report from the Pew Research Center.

"We are the leaders in a growing market," Sandberg said. "Do I think there's time and room for people to do a lot more sharing before we reach the zero sum game? Yes." Facebook shares have dropped 39 percent since an initial public offering last May, leaving Facebook with a market value of about $56.3 billion.