I sort of built something like this and i can give you all the advice in the world but it looks like you are on the right path. You are going to have a chicken

I sort of built something like this and i can give you all the advice in the world but it looks like you are on the right path. You are going to have a chicken and egg problem, I don't know if you've attempted this solution yet but you can lower the adoption friction of new users by letting them tweet. Then you can use the twitter api to, say, find all of the tweets with the #shoutt hashtag and process them. This way you can make use the the massive platform that is Twitter and users don't even need to signup or install an app. Good luck.