I am using Xcode 7.0, testing on iOS 9.0.2 and using Facebook SDK 4.7.0.
When I am logging in a user, most of the time everything works as it should, however sometimes I
For Xcode8 - iOS10,
Enable Keychain Sharing within Capabilities tab of target fixed my issue.
More details can be found here : https://github.com/facebook/facebook-sdk-swift/issues/51
For Xamarin Studio (Suggested by @Kenneth),
Add the Entitlements.plist file to Custom Entitlements under the iOS Bundle Signing options in the iOS project.