<fb:login-button perms="user_likes">
  Grant Permissions to Allow access to Likes
</fb:login-button>

<button onclick="checkDoesLike()">Check si eres fans</button>

<h1>Like this Application's Page</h1>
<fb:like-box profile-id="ID_APP"></fb:like-box>

<script>
window.checkDoesLike = function() {
  FB.api({ method: 'pages.isFan', page_id: 'ID_APP' }, function(resp) {
    if (resp) {
      alert('You like the Application.');
    } else {
      alert("You don't like the Application.");
    }
  });
};
</script>

Verifica si el usuario es fans de la pagina o no.

RECONOCER SI EL USUARIO ES FAN DE TU PAGINA FACEBOOK

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JavaScript