According to PiperJaffray's latest report on the preferences of American teens, Instagram has leapfrogged Facebook and Twitter in popularity among teenagers.
According to the report, 30 percent of U.S. teens named Instagram the most important social network, while Facebook and Twitter were chosen by 23 and 27 percent, respectively. Especially Facebook has recently lost popularity among younger audiences. A year ago, 33 percent of teens had named it as their favourite. Many saw Facebook's acquisition of Instagram as an attempt to re-connect with the youth. If that were the case, choosing Instagram appears to have been a good choice.
You will find more statistics at Statista