What is your favorite growth hacking trick?

People hate calls. People hate emails. People hate mail. Do you really want your first impression to be that of an interloper and a pusher? Then again, most recipients aren't event going to look at what you send them. What is your niche? Office managers for private family healthcare providers in Peoria? Athletics department directors for NAIA schools? Sales managers at wholesale car dealers that make over $180 million per year in gross revenue? Know your niche and define your buyer (and it better be the CIO or VP). Is your buyer female or male? Older, middle age, or younger? What about her or his college education? What does he drive? Where does he live? Where does he eat his lunch and get his coffee in the morning? What does he read? Etc. Go to your buyer. Find congregations of your buyer. Professional associations. Conferences. Meet-ups. Trade shows. Offer to do free presentations--not on your product but on best practices or trends you observe in the industry. Make your presentation about solving problems your buyers deal with every day. Write blogs or columns for media they read. Again, focus on what they need/want to read. You will have a hard time keeping enough business cards in stock and click-throughs from your byline. This is a true "targeted outreach campaign." Don't waste your money and time with anything less than this. You're going to do great. Please let me know if you'd like to talk about it more!

Retention - if you build something people want/use AND come back and use often, then you can usually figure out a business model to make it work (if there's a big enough market).

I'm answering your question assuming that you hope to be able to replicate it's own success in your own mobile app. There are a couple of factors responsible for it's growth that are instructive to anyone building a mobile app. "Leveraging the intimacy and privacy of the mobile phone." We now have an *intimate* relationship with our phone like no other device in the history of technology. Every internet company that started before around 2010 has built their core interactions around "the old web" one which was accessed primarily via a browser on a computer. Companies that start with a clean slate, should be building their interactions around how to do whatever the app is supposed to do while leveraging what is unique to people's relationship to their mobile devices. Photo-sharing has become a core part of the way we communicate now. Snapchat built something that provided an experience that leveraged the feeling of privacy and intimacy that is unique to mobile. "Provided an escape from the "maturity" of other online services." Too many parents, aunts, uncles and other "old people" have encroached into the social networks of teens and young people. As a result, they've had a desire to find places to express themselves in places inaccessible by older generations. An important distinction is that it's not just parents and relatives that young people are trying to avoid, but also employers & colleges who are increasingly using "mature" social networks to review applicants. "Leveraged PR even bad PR" The fact that the app got so much press about it being used to sext was perfect PR for the company, as it essentially reinforced the brand experience that it has today. Essentially, "if it's safe enough to send a sext, it's safe for any kind of communication I want to have." And although the safety and security of Snapchat is actually not as advertised, it still enjoys the reputation of having less impact than any primarily web-based service. Building a successful mobile application is one of the hardest challenges to face designers, programmers and entrepreneurs in the history of writing software. Happy to talk to you if you're considering building a mobile app, about what I've learned about the "table stakes" for success.

Before you spend $ or time marketing your service / website, ensure that your brand itself is quite strong. For example, if your name or domain is awkward, ambiguous, or off-putting, then every dollar and hour you spend promoting your brand will work less efficiently in your favor than if you were spending the same amount of money and time marketing something more robust and attractive.

For the sake of getting your question answered here it would help to simplify but that aside without firm stats on engagement and sharing it would be difficult to say however with the paid expansion of game like Flappy Bird, Angry Birds I would say viral apps come close to if not over a k-factor of 1.