You can use this library located at Github.
<?php
require './facebook.php';
$facebook = new Facebook(array(
'appId' => 'YOUR APP ID',
'secret' => 'YOUR API SECRET',
'cookie' => true, // enable optional cookie support
));
if ($facebook->getSession()) {
PLACE YOUR WP LOOP HERE
} else {
echo '<a href="' . $facebook->getLoginUrl() . '">Login</a>';
ADD YOUR APP's 'Connect' button here.
}
?>
Include facebook.php from the library located at Github (2nd line).