A lot of sites like this don't like you to have a SSL decrypt rule in place. I have had the same problem with Dropbox and in the end I had to put a no-decrypt rule in specifically for the people who use that service. Basically, the site is saying that it doesn't like you doing something in the middle of the transmitting, like SSL decryption. Those sites will generally give you a reduced webpage much like what your screenshot showed. Try going to your Decryption rules and adding one that says, Any source, any destination, URL category as only Facebook, no decrypt and ssl-forward Proxy. If that works you may want to restrict it down to specific sources or specific destination zone.
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