Open Graph validation for HTML5
For HTML5, add this to your html element like described on ogp.me and keep your og: prefixed properties:
<!doctype html>
<html prefix="og: http://ogp.me/ns#">
<head>
<meta property="og:type" content="website" />
...
For XHTML (like OP's question), use the name attribute instead of property attribute. Facebook lint will throw a warning, but the meta value will still be recognized and parsed.
<meta name="og:title" content="Hello Facebook" />
Yes. To validate as HTML5, add the prefix attribute from the Open Graph docs:
<!DOCTYPE html>
<html prefix="og: http://ogp.me/ns#">
<head>
<title>Valid HTML5!</title>
<meta charset="utf-8"/>
<meta property="og:title" content="">
</head>
<body></body>
</html>
Copy and paste the above to the w3 validator to check.
It is production ready – Apple uses this method on apple.com.