My daughter is 13....I would add a couple of thoughts. Tumblr is can also be viewed as a 'community' site. My daughter has friends from around the world formed on Tumblr around common interests (mostly music and television).
Also, many her age (moving to high school next year) are already over Facebook. This is going to be a huge problem for FB.
Having an iPhone is NOT a status symbol, but rather a necessity because of the lock in of iMessage and FaceTime. SnapChat is the first dent in that armor and a service that can displace that lock in will also be a huge threat to Apple.
And yes, SnapChat is NOT a sexting app. Everyone uses it. Its how they would act if they were home alone with friends without supervision: being kids.