Snapchat is a photo sharing app. The story was the growth hack: To help foster the sense of privacy and security that wasn't readily available from Facebook or Instagram. "Snapchat creates a brand new way to communicate with friends that is fun and interactive, while creating a sense of privacy through the disappearing nature of the content. We know that the security isn’t foolproof—but it’s far better than its predecessors."
Once it started being used for sexting, mainstream media started mentioning Snapchat - and Poof! that kind of publicity just generates curiosity and interest that turns into a haven of word-of-mouth.
It's kind of like the curiosity and publicity that Google generated for its Fiber project in Kansas City. It has been four years since the announcement by Google. Just How Big is Google Fiber's Impact? The goal was mainly to light a fire under the Duopoly to deploy Gigabit broadband. Amazon is probably grateful as well. They need fat pipes to keep cloud services, kindle sales and their video service pumping into homes and businesses across America.
Target just launched a digital movie service as well, so those pipes are going to be mighty full.