I can't speak for how active they both were or weren't on SM (LG at least has been shown to have had several SM accounts however), but in terms of what the draw is for Snapchat, it's primarily just a way for this age group to stay in contact with each other. Basically takes the place of texting (source: I have several tweens/teens of my own
). The photo element of it is kind of extraneous, and more of a way to talk "face to face". The photos generally aren't ones people would want to save.
So whereas people in my age-group tend to use texting to quickly communicate, this age group uses Snapchat. And another upside to some is that it's NOT saved. Communications are quick and private. The privacy isn't all that important to a lot of them, but to some it would be I think.
MOO.