At CloudRail we just released our unified API integration SDK for Xamarin, and I thought it would share it here.
With the SDK it's easy to implement popular social, cloud storage, messaging, location and payment APIs. And we are bundling APIs of the same kind into one universal API, so you handle e.g. social logins from Facebook, Twitter, Google etc. the same way. The same goes for cloud storage and the other API categories.
Our iOS and Android SDKs have already been implemented in many applications, but we got a lot of requests for a for Xamarin SDK, so we decided to develop it. I hope you will find it useful, and of course it's free to use (paid plans does exists, but not required).
Read more about the new SDK here: Unified APIs for Xamarin
Hope you find it useful!