Most companies don't disclose their launch strategies, for obvious reasons, so the following is opinion/conjecture based on my own experience:

Part of Secret's success was releasing a novel app that had some contextually relevant cultural value at the time of it's release--the idea of anonymity in the digital social space. Mix that with getting the app into the hands of target-relevant influencers, a powerful PR campaign and a little bit of luck that it got picked up by the tech media.

Though in the end, due to a quick growth on an idea that had not been really properly vetted and tested secret turned into something ugly:

http://www.dailymail.co.uk/sciencetech/article-3064077/The-secret-s-Anonymous-app-shuts-following-legal-battles-claims-encouraged-bullying.html