title: What is the correct implementation of the Open Graph "article" type?
body: <p>I need to add Open Graph tags to a blog page. It seems from reading the spec (<a href="http://ogp.me/" rel="noreferrer">http://ogp.me/</a>) using an <code>og:type</code> of <code>article</code> is the way to go. However, I find the spec unclear and I'm unsure how to implement the syntax correctly.</p>
  
  <p>Two example websites implement this differently for example:</p>
  
  <ol>
  <li><p>Example from GitHub: (<a href="https://github.com/niallkennedy/open-graph-protocol-examples/blob/master/article-utc.html" rel="noreferrer">https://github.com/niallkennedy/open-graph-protocol-examples/blob/master/article-utc.html</a>)</p>
  
  <pre class="lang-html prettyprint-override"><code>&lt;head prefix="og: http://ogp.me/ns# article: http://ogp.me/ns/article#"&gt;
  &lt;meta property="og:title" content="..."&gt;
  &lt;meta property="og:type" content="article"&gt;
  &lt;meta property="article:published_time" content="..."&gt;
  </code></pre>
  
  <p><em>Note the <code>og</code> and <code>article</code> namespaces are registered and that <code>og</code> and <code>article</code> are used as properties.</em></p></li>
  <li><p>BBC News article</p>
  
  <pre class="lang-html prettyprint-override"><code>&lt;head&gt;
  &lt;meta property="og:title" content="..."&gt;
  &lt;meta property="og:type" content="article"&gt;
  &lt;meta property="og:article:author" content="..."&gt;
  </code></pre>
  
  <p><em>Note no namespace registration and <code>og</code> and <code>og:article</code> are used as properties.</em></p></li>
  <li><p>A variation I've seen in the wild of the above, registering only the <code>og</code> namespace and still referencing <code>og:article</code> as a property.</p>
  
  <pre class="lang-html prettyprint-override"><code>&lt;head prefix="og: http://ogp.me/ns#"&gt;
  &lt;meta property="og:title" content="..."&gt;
  &lt;meta property="og:type" content="article"&gt;
  &lt;meta property="og:article:published_time" content=".."&gt;
  </code></pre></li>
  </ol>
  
  <p>Option 3 is what I used the first time I tried to implement this. When I ran it through the Facebook validation tool I was told:</p>
  
  <blockquote>
    <p>Objects of this type do not allow properties named 'og:article:published_time'.</p>
  </blockquote>
  
  <p>For the moment, I have gone with option 1 and although this validates, I would like to know what the definitive correct syntax is?</p>