Started and recently published DrinkControl for Android http://drinkcontrolapp.com Taking into account that iOS version brings just about $50/month, no illusions about becoming profitable ever. Still was worth it for understanding Android as development platform and its ecosystem (biggest surprise so far - beeing in Samsung app store is more important than beeing in Google Play).
Start with closest evironment- your friends, collegues, people you know. Let them know what you built, invite them, may be ask a favor to come and register.
I remember how founder of CollegeHomour shared his experiences how he started site by creating countless accounts by himself and posted basic content to make site rolling. You should start with some user base to make site rolling.
What from technical point of view- sign in twitter and facebook icons looks wrong. You should make Twiiter/Facebook auth as first choice.