When Facebook first launched the Open Graph protocol they used the domain opengraph.org and you added the following to your site:
<html xmlns:og="http://opengraph.org/schema/">
In what looks like an admin slip up, the domain is no longer registered to Facebook are is currently parked by Go Daddy. This means a lot of older tutorials and sites now point to the wrong domain. The Open Graph documentaion currently sits on ogp.me and you should be using:
<html xmlns:og="http://ogp.me/ns#">