The Facebook crawler was added in a recent update, you will need the new data file before it will be identified properly.
You can download the latest data file from here
, you should place it into your website's App_Data folder. Then modify your web.config to contain this line in the <fiftyOne> element (there will probably already be a line like it commented out).
You may then need to restart your site's AppPool.
I hope this helps,