Is there a way to retrieve the list of Facebook users that have clicked the like button on an external website?
FQL Should do the trick.
$facebook->api_client->fql_query('SELECT user_id FROM like WHERE object_id="OBJECTID"');
Here is the link.
Some general info about FQL.
FQL is initiated using the JavaScript SDK, in this way.
If you can post some sample code-I can try and give some more specific help.
A note should be made-once you've got the user ID's, you can just query the names and get them in a list.
EDIT: To get the URL of an object you should query this table using fql.