Assuming you mean node/xxx..., then use http://api.drupal.org/api/drupal/includes!menu.inc/function/menu_get_object/7 in your block code.
 
Nancy
Injustice anywhere is a threat to justice everywhere. -- Dr. Martin L. King, Jr.

From: toon severijns

I'm looking for a way to set a facebook page on a per node basis (i.e. when editing the node) and display it in a block next to the node. 
How would i go about this? How can i set the facebook plugin based on the node field holding the facebook reference?