fb custom plugin login problem

Hi I created a small fb login plugin as the usual ones do not work as the users are also registered in a separate database. My problem is suddenly if any user logs in they automatically login as this 1 user, and have access to his page and cannot login with their own fb “account” to their page.
I hope someone can help – thank you in advance
Below is the code:

<?php
/**
 * Plugin Name: fb_login
 * Description: Extends wp-login to use Facebook login.  Uses email registered with fb to log in to WordPress.
 * Version: 1.0
 * Author: Mel
 */

function javascript_init() {
	//if ($redirect_to == ''){$redirect_to = site_url();}
	if ($requested_redirect_to == ''){$redirect_to = site_url();}else{$redirect_to = $requested_redirect_to;}
?>
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.6.2/jquery.min.js" type="text/javascript"><!--mce:0--></script>
	<script>
  window.fbAsyncInit = function() {
    FB.init({
      appId      : '699853586802508',
      xfbml      : true,
      version    : 'v2.5'
    });
  };

  (function(d, s, id){
     var js, fjs = d.getElementsByTagName(s)[0];
     if (d.getElementById(id)) {return;}
     js = d.createElement(s); js.id = id;
     js.src = "//connect.facebook.net/en_US/sdk.js";
     fjs.parentNode.insertBefore(js, fjs);
   }(document, 'script', 'facebook-jssdk'));
</script>

    <script type="text/javascript">

	function login_to_wp(email)
	{
		$.ajax({
		  type: "POST",
		  url: "<?php echo plugins_url().'/fb_login/log_user_in.php'; ?>",
		  data: ({user_email: email, path: "<?php echo ABSPATH; ?>"}),
		  success: function(msg)
		  {
			  if (msg == 'true')
			  {
				 window.location.href = "<?php echo $redirect_to; ?>";
			  }
			  else
			  {
			     document.getElementById('fb_login_message').innerHTML = 'There is no user with email: '+email;
			  }
		  }
		});
	}
	</script>

    <?php
}

add_action('login_head', 'javascript_init');

function fb_message()
{
	return '<div id="fb_login_message"></div>';
}

add_filter('login_message', 'fb_message');

function facebook_login() {
	$url = site_url();
	?>   

    <div class="login label">Connect with:</div>
    <div class="buttonspace"><div class="fb-login-button" data-max-rows="1" data-size="medium" data-show-faces="false" data-auto-logout-link="false" data-scope="public_profile, email" onlogin="checkLoginState()"></div></div>

    <div id="status"></div>

    <script type="text/javascript">

	  function checkLoginState() {
		  FB.getLoginStatus(function(response) {
			statusChangeCallback(response);
		  });
	  }

	  function statusChangeCallback(response) {

		  if (response.status === 'connected') {

			 FB.api('/me', function(user_mail){
					  email = user_mail.email;
					  login_to_wp(email);
			 });
		  }
		  else
		  {
			  document.getElementById('fb_login_message').innerHTML = 'Could not log in to Facebook';
		  }
	  }
</script>
<?php
}

add_action( 'login_form', 'facebook_login' );
?>