In my page, I have this:
<meta property="og:image" content="<?php echo $picURL; ?>"/>
Which when executed is rendered like this:
<meta property="og:image" content="http://a3.sphotos.ak.fbcdn.net/hphotos-ak-ash3/556898_400257580012798_100000856787624_1059515_311974781_n.jpg"/>
But the Facebook scraper is seeing it like this:
<meta property="og:image" content="">
It seems it is not considering images from Facebook.
See my answer here but you can't have images hosted on Facebook set as your meta tags. It used to give you a clearer error message that you couldn't hotlink Facebook images.