https://github.com/sharker86/AdsPCLXamarin
This is an example that how add ads for your aplications with Xamarin.Forms and Customs Renderers.
The ads are implemented with admobs {windows phone}, google play service {Android} and iAds {iOS}.
Here, I explain a briefly how we do it
http://thelostworldofsharker.esy.es/programacion/11-ads-in-xamarin-with-customrendereds-2