I noticed that when my iMac's display goes to sleep while Safari is showing Facebook (i.e. Safari is the current active app and its active tab is Facebook.com) I am unable to turn the display back on. I have to unplug my iMac and restart it. Is this happening to you? Do you have any solutions (other than making sure you don't have Facebook running)? Have you experienced this with other browsers?
iMac 27", Late 2009 : OS X Mavericks 10.9