AngularJS service not returning a promise

I recently started using AngularJS, although I can see some improvements, I can't get my head round the promises. I created a login service that uses Parse framework to enable users to register and a register controller that sends the data from a form to the service. Since signing up takes a few seconds I decided to use a promise to return the current user back to the login controller once he has been signed up. Unfortunately I keep getting this error: Cannot read property 'then' of undefined. Could someone please point me in the right direction?

Controller:

(function () {
'use strict';

angular
    .module('app.register')
    .controller('RegisterController', RegisterController);

RegisterController.$inject = ['$q', 'logger', 'loginservice'];
/* @ngInject */
function RegisterController($q, logger, loginservice) {
    var vm = this;
    vm.title = 'Register'; 
    vm.register = register;     
    vm.user = {
        username: "",
        password: "",
        email: ""
    };

    activate();

    function activate() {
        logger.info('Activated Register View');
    }

    function register() {
        var promise = loginservice.register(vm.user.username, vm.user.email, vm.user.password);

        promise.then(function(userData) {
            console.log("User: ", userData);
        });
    }
}
})();

Service:

(function() {
'use strict';

angular
    .module('app.core')
    .factory('loginservice', loginservice);

loginservice.$inject = ['$http', '$q', 'logger'];
/* @ngInject */
function loginservice($http, $q, logger) {
    var service = {
        // logIn: logIn,
        // logOut: logOut,
        register: register
    };

    return service;

    function initialize() {
        console.log("parse initialized");
    }

    function register(username, email, password) {
        //alert("HELLO FROM " + username + " " + password + " " + email);

        Parse.initialize(appId, parseId);
        var user = new Parse.User();
        user.set("username", username);
        user.set("password", password);
        user.set("email", email);

        $http.get(user.signUp(null, {
         success: function(user) {
            return user;
          },
          error: function(user, error) {
            return error.message;
            // Show the error message somewhere and let the user try again.
            alert("Error: " + error.code + " " + error.message);
          }
        })).then(function() {
            return user;
        });
    }
}
})();