What's the best way to block this entirely? /etc/hosts won't work because it's not on a subdomain (the iFrame loads from http://www.facebook.com/plugins/activity.php). This would make a nice Chrome/Firefox extension.
It has lots of insightful and useful information about identifying and controlling Enterprise 2.0 apps (Facebook, Twitter, Skype, SharePoint, etc.)
It seems to have removed all Open Graph stuff from the web for me.
A greasemonkey script to always click 'No Thanks'.