Personally I would've turned down the offer too. The owner is probably living a great life right now with his millions and doesn't see the need to sell his app. I'd value my extremely popular app more than $3 billion to be honest. Money means a lot, but when you have enough money then it isn't really a priority.. for me of course. I guess the owner must be making a lot of money somehow from the app to turn down $3 billion though.
It's popular because its easy to function and got lucky with word-of-mouth advertising. The name Snapchat is very attractive and it's not that hard to use, from what I've heard and seen (you'll never find me using it ;p). I guess that people want to be alerted when a new photo is sent, because it is a live chat I assume.
I don't know what you mean by that last statement but I thought being alerted would be better than not being alerted. People would want to know when they have received a new photo. Its pretty much a live chat but with photos, and you get alerted in chats e.g IM