Snapchat is based on a lie. You can't give someone information and also control what they do with it, to make sure they delete it. It's the very definition of a passing fad, it doesn't do anything unique or special not done many times better by any other instant messaging program. It makes sense then that its growth would slow, because after you realize you can't rely on its singular selling point (the ephemerality of messages you send to other people), what's left is a crappy version of any other program that lets you share pictures with people.