Whenever I build apps, along with giving best possible product, my focus is always on K-factor (http://en.wikipedia.org/wiki/K-factor_(marketing)).

Viral Factor, K = No. of invites X Conversion rate

As long as, K >1, your app is growing. And best way I figured out to do that was to make people tweet and make that tweet provoking enough to let her users click on that tweet. Example, I encourage people to boast about number of repins or followers they got because of Pinwoot and tweet reads something like : 1) I got 954 repins on #Pinterest using @Pinwoot. Add your pin here https://pinwoot.com

2) Try @Pinwoot.com to get free followers on #Pinterest https://pinwoot.com

Tweet Copy 1 performed 267% better than Tweet Copy 2 in getting new users.

Don't forget viral cycle: http://www.quora.com/Viral-Growth-and-Analytics/What-is-the-...