I'm studying a master on mobile marketing and one of the things they taught us is how does monetization work. To put it simple, if your app doesn't have enough traffic, it won't be profitable to go through all the ad implementing issues. It will just make you lessen your downloads due to a "bad" UX. Even though, if you think your app is ready, then go along and implement an ad network SDK into your app, it could be AdMob's, Chartboost... Lately though, there's been an indie community that monetizes connecting to more than 30 ad network, resulting in better CPM's, they're called Tappx, and they're a start up that really seems to be worth investing in haha From what I talked with them, they just do that with people with enough traffic to monetize and be profitable. Same story as always. Anyway, hope my small explanation helped you in some way.