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).
<detection name="data" binaryFilePath="~/App_Data/51Degrees.mobi-Lite-2012.01.18.dat" enabled="true" />
You may then need to restart your site's AppPool.
I hope this helps,
Tom