As a couple of others have pointed out, that age group seems to have abandoned Facebook. My nieces and nephews are 12-15 and they hardly ever sign onto Facebook anymore. Snapchat is where it's at. My guess is that the girls used the app that they're most comfortable and familiar with. They may not have ever used Facebook live before. Facebook live was created as a competition for Snapchat. If you were already using Snapchat to begin with, you might not go back to the other. Plus, if they knew all their friends utilized the other app more, it would make sense for her to have gone with the app that she thought would reach the most people.
With that being said, it's hard to say what happened. I am sure she did it very quickly. Her finger might have just hit the button/app it came to first. I find Facebook live not as user friendly, myself. You have to open Facebook, bring up a post, and hit the "live" button. My snapchat and Instagram apps, however, are accessible with a single swipe of my finger.