Recently we have some cases that the customers reported the rendering issues with Facebook after upgrading to IE9 RTM.
In every case Facebook had been added to the Compatibility View list due to issues during the Beta or RC or IE8. Facebook does not want to be on the CV list, and we don’t want them on the CV list.
To get back to a great rendering experience, please do the following:
ALT + Tools :: Compatibility View Settings
Remove Facebook.com from the list
That should put Facebook back in IE9 mode.