let credential = FacebookAuthProvider.credential(withAccessToken: FBSDKAccessToken.current().tokenString)
Auth.auth().signIn(with: credential, completion: {
                            user, error in
                            if error == nil {
                                // Sign up successful
                                // Save user's selected avatar image to our Firebase storage
                                let firebaseStorage = Storage.storage().reference()
                                let avatarImageReference = firebaseStorage.child((user?.uid)! + ".jpg")
                                // Download profile picture from Facebook URL
                                ImageDownloader.default.downloadImage(with: URL(string: profilePictureUrl.stringValue)!, options: [], progressBlock: nil, completionHandler: {
                                    image, error, url, data in
                                    let _ = avatarImageReference.putData(data!, metadata: nil) {
                                        metadata, error in
                                        if error == nil {
                                            // Success
                                        }
                                    }
                                })
                            }
                        })