Using the pidgin facebook plugin, I'm able to chat on facebook without the browser, but since the plugin uses http anyway, it will still be blocked. The trick that I'm using is that the User agent used by the facebook plugin is Opera, whereas I use firefox. So, I changed my iptables rule to match the user agent as well as the destination port.
iptables -A OUTPUT --protocol tcp --dport 80
-m string --string "User-Agent: Mozilla" --algo=kmp
This rule will block me from going to facebook when I've turned off access to the internet, but still allow me to chat.