You're referencing Xamarin.Auth in `..\..\..\` but it should actually be in `..\libs\Xamarin.Social\Xamarin.Auth` as a submodule.

To fetch submodules, you'd need to run `git submodule init`, then `git submodule update` in `Stampsy.Social` root directory, then go to `libs/Xamarin.Social` and run the same two commands from there.

Same goes for all project references to `Xamarin.Social` and `Xamarin.Auth`.