Instagram recently has announced that it now has 300 million active users, sprinting past rivals that include Twitter. The Facebook-owned social media site added 100 million users in the last nine months alone and has quickly become the must-use app for teens and preteens, with 76% of teens using it regularly (according to a survey by Piper Jaffray). Along the way, it's become a prime tool for users to measure their popularity and feed their anxieties, monitoring "likes," analyzing comments, etc. "That's not what the app creators intended, of course, but it does make psychological sense," writes Time's Rachel Simmons. "As they become preteens, research shows that girls' confidence takes a nosedive. Instagram, then, is a new way for girls to chase the feeling of being liked that eludes so many of them. Instagram becomes a popularity meter and teens learn to manipulate the levers of success."