The only way I could replicate the error was by putting a question mark after your Facebook ID. For example, on the settings page for the plugin, I entered the following into the Facebook ID box:
172343776306627?
After, I clicked "Save Changes" I got the exact error you are getting when I viewed the widget. Can you make sure there are no extra characters mixed in with your Facebook ID on the settings page for the plugin. The only thing that should be in the Facebook ID box is the following:
172343776306627