Photographer: Geber86 via Getty Images

Facebook Inc. acquired popular teen mobile app TBH, which lets users anonymously answer questions about their friends. Terms of the deal weren’t disclosed.

The app, whose name stands for To Be Honest, will operate separately from Facebook, similar to how the social network handled its acquisition of photo-sharing app Instagram, Facebook said. TBH was launched on Aug. 3 and already has more than 5 million users who have sent more than a billion messages -- and for now is only available for iPhones, and only in some U.S. states.

Facebook has been seeking new avenues to reach flighty teenage users, who want to communicate in different ways and who may view the flagship Facebook app as more of a stodgy utility than a place to have fun. The social-networking company has tested and shuttered several game-like apps targeted to young people, after being rebuffed in a bid to purchase Snapchat years ago.

TBH works by giving users a description, such as "always has the best manicure," then letting them anonymously choose which person meets that description from a set of four names in their network of friends. By keeping the activity structured, TBH hopes to maintain positive conversations, instead of regressing into bullying as other anonymous apps have done.

“TBH and Facebook share a common goal -- of building community and enabling people to share in ways that bring us closer together," Vanessa Chan, a Facebook spokeswoman, said in a statement. “We’re impressed by the way TBH is doing this by using polling and messaging, and with Facebook’s resources TBH can continue to expand and build positive experiences.”

Still, it’s unusual for Facebook to take interest in an app that promotes anonymous activity. Facebook’s advertising business model stems from its rule requiring people to use their real identities on its network, making clear what they like and don’t like and attaching their internet behavior to their name.