If you want long term evangelists you're going to have to think outside of the relationship between your product and the customer and focus specifically on the customer.
Identify the influencers who are already engaging your brand through social media channels and see what you can learn about them.
If their profile picture is them playing an instrument, you might send then a gift card to Itunes or Guitar Center.
In Short: Spend a little money and show your interest in them instead of just rewarding their interest in you.
I had Wheat Thins send me a box of crackers and that was cool but I sent one tweet out and was done. General Electric saw that I was wearing a Batman shirt and sent me a really cool book as well as a hand written thank you note.
I regularly engage and promote content about GE.
I think the answer to this is very much about customer engagement. It's all about speaking directly with your customers and really listening to them. If you are prepared to listen and react to them, they are often more interested in reciprocating.
You've also not got to be afraid to guide them if you want them to talk about your product. Suggest to them that if they like something that you've done that they share it on Facebook, LinkedIn or other social site.
One of the best forms of engagement is a great customer support department, they are on the front line talking with customers daily and can really be your best brand advocates.
A good way to start would be to first define metrics to identify who are your happy users, and engage with them to let them know you care about adding value to their experiences.
For simplicity, start by looking at your most engaged users (e.g.:spent over 10 minutes per day in your app, for 5 out of the last 10 days). Reach out to these users to understand (a) what they like about your app, and (b) how the app could delight them further. Offer them a promotion/reward for helping you with feedback. They will see you are keen on keeping the relationship going and feel part of any success you have.
The same could also work for your most unhappy users. Ping users who aren't engaging with you, have left you bad feedback, to find how you can make yourself relevant to them, and what they are missing out on. You may be surprised to see how, once you start listening to them, they can become your most vocal evangelists.
Identify your top 5-10 most engaged users and help them become more involved with the company as brand ambassadors offering perks that make them feel valued and appreciated.
In my experience, we have been able to grow our travel company mostly via word-of-mouth by focusing our resources on those clients that have shown high interest in becoming more involved with the product/service.
Happy to chat further about this.
Good luck and have fun with it!