Hey Mike,
I was about to say that I don't think Facebook can download podcasts but just to make sure I went into our auditing tool and found the following user agent.
facebookexternalhit/1.0 (+http://www.facebook.com/externalhit_uatext.php)
There are 4 requests from this user agent within the last month. The next version of the statistics system will include an export option that will include the user agent information. Will this allow you to drill into the unknown clients, or clients that are not within the top ten?