Best answer I've found: "After doing a little research, it seems that it's a Facebook problem that many many people are having as well and Facebook isn't fixing. The majority of them have changed to Firefox (which i would recommend anyway as it is a much more secure browser) or Google Chrome. One person posted in a workaround of just selecting and dragging from the top of the page down which will cause it to scroll but that is by no means a fix. Not sure what else to say other than be sure that you have all the latest Windows Updates."

From: http://www.justanswer.com/computer/2nx8r-scroll-bar-missing-when-using-ie8-facebook-only-help.html