Just one point: this is an app idea that has been executed many, many, many times before. I could find you a half-dozen other examples if I had more time for Googling “self-destruct message app” right now.
Nothing against SnapChat! If people like it, that’s great. But it’s a good example of software success clearly driven by cultural factors rather than the inherent attributes of the app itself*. This distinction is very rarely made when people write about software fads, but it’s important.
* Sure, you can tell a story about design/business model/whatever. I don’t think I buy it.