Can anyone tell me the benefits of using fbcdn.com instead of cdn.facebook.com? I've seen a lot of sites using secondary domain names to control their CDN and wasn't sure the benefits of that or the inefficiencies of just using sub-domains.

This is not my area of expertise so this may be 100% wrong, but my understanding is that the separate domain is pointed at a completely different DNS entry where as a subdomain still uses the same DNS and is routed per the subdomain.