Based on the WWW-Authenticate
header in the response I'm assuming you're using the .NET backend for the mobile service. Login with the token from access providers (Facebook / Microsoft) is still not supported in the .NET backend. If you use a mobile service with the node.js backend, then you should be able to use that code on the client-side to login to the mobile service.
Support for client-side login for those providers should come in the near future for the mobile services .NET backend.