Save this as an include - for example, as /_includes/og.html.
Document Head
Call the include in the document head:
Gotchas
Facebook generally needs URLs - which must include ‘http’ or ‘https’ (rather than a relative link) - hence the use of site.url throughout rather than site.baseurl.
Facebook Debugging
You can have Facebook scrape your page using the Developers debugging utility. This provides lots of useful information regarding how Facebook views your document.