Go to /themes/{your_theme}/header.tpl (line 28) and

Need change this:

html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en"

to:

html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en"
xmlns:og="http://opengraphprotocol.org/schema/" xmlns:fb="http://www.facebook.com/2008/fbml"


Do not forget to clear the cache.