title: Facebook API: undocumented 'properties' attribute on Post type
body: <p>The official API docs are too sparse (<a href="http://developers.facebook.com/docs/reference/api/post/" rel="nofollow">here</a>). There is at least one attribute not listed there which you can use very usefully when making posts. This is a self-answered question to document the 'properties' field because I could not find all the information on SO.</p>