// Fonction hypothétique...
function getFacebookLikeCount() {}

var currentLikeCount = new ReactiveVar();

Meteor.setInterval(function () {
  var postId;

  if (Meteor.user() && postId === Session.get('currentPostId')) {
    getFacebookLikeCount(Meteor.user(), Posts.find(postId).url, function (err, count) {
      if (!err) {
        currentLikeCount.set(count);
      }
    });
  }
}, 5 * 1000);

// Insert in "post_item.js".
Template.postItem.helpers({
  likeCount: function () {
    return currentLikeCount.get();
  }
});

// Note : similar feature for dictionaries in package "reactive-dict".