Instagram usage reached a new peak among US teens.Didem Mente/Anadolu via Getty ImagesInstagram is the most used app among US teens, a new Piper Sandler survey found.TikTok was the second most used, and the most liked.Snapchat dropped off in both popularity and use among teenagers.Teens are using Instagram like never before.According to the investment bank Piper Sandler's latest survey of US teenagers, Instagram is the most used social-media app (and TikTok is the most liked).Piper Sandler surveyed about 13,500 US teenagers this fall to understand their behaviors on things like social media, fashion, and food.The analysts found that 87% of teens said they used Instagram at least once a month.