I came across this thread while using the facebook debug tool and trying to find the cause of the wrong product images being pulled. I have been adding additional og tags that FB debug was complaining were missing. For the og:url I added <?php echo $canonicalLink; ?>.

Code:
<meta property="og:url" content="<?php echo $canonicalLink; ?>" />
Facebook accepts that ok. Is there any reason I should not do this?

For the same reasons mentioned above I have encountered the same message

The following will be treated as a redirect by the crawler:

A HTTP redirect
A <link rel="canonical" href=".." /> tag
A <meta property="og:url" content=".." /> tag

The final URL, which we tried to extract metadata from is highlighted in bold.
I did notice though that on another site using the ceon uri mapper mod the http redirect is not an issue in FB debug because canonical writes the url as it appears in the address bar. Just thought I'd mention it. Still haven't gotten tot he bottom of wrong images being pulled.