Where have the Facebook likes gone on my website?

When looking at our homepage today. I was surprised to notice that all the Facebook likes had gone.

Particularly strange because we had done nothing that could possibly have broken it.

So whilst getting to the bottom of it, I wondered if things had changed at Facebook regarding the way URL's work. Technically speaking these are considered different domains:

So I gave it a whirl and changed the URL for the Facebook like button and hey presto! The likes are back.

Pretty short post but we would have saved some time if we had found it.

Post type: 
Non technical
Drupal version: 
Drupal 6 (D6)
Drupal 7 (D7)