Go to your blog HTML edit tab, and search for <head> code and below it add this code
<!-- facebook open graph -->
<b:if cond='data:blog.pageType == "item"'>
<meta expr:content='data:blog.pageName' property='og:title'/>
<meta expr:content='data:blog.canonicalUrl' property='og:url'/>
<meta content='article' property='og:type'/>
</b:if>
<b:if cond='data:blog.postImageUrl'>
<meta expr:content='data:blog.postImageUrl' property='og:image'/>
<b:elseif cond='data:blog.postImageThumbnailUrl'/>
<meta expr:content='data:blog.postImageThumbnailUrl' property='og:image'/>
</b:if>
<b:if cond='data:blog.metaDescription'>
<meta expr:content='data:blog.metaDescription' name='og:description'/>
</b:if>
<meta expr:content='data:blog.title' property='og:site_name'/>