Facebook FQL - >'page'表 - >'page_id'参数

Where can I find detail information about that parameter?

I can't understand how is parameter linked with real page and where it is generated...

So, in my case I make facebook-like button on some page of my website and want to get count of likes for that page, but I dont know that page_id:

$result = $facebook->api(array(
  'method' => 'fql.query',
  'query'  => 'select fan_count from page where page_id=!!!???!!!'
));