Facebook has finally started deleting the images from their server within the 30 days time-limit. well it still seems to be a long period of "I will have to make sure no-one has the direct link to my image" thought, but it is way better than its previous practices.
If you must know, Facebook has been keeping the images in their CDN servers for years before they can actually be deleted from their Data retention technology. Twitter on the other hand, never found to be keeping their users private contents on their server without their knowledge.
You probably don't want to show your boss, a naked image of your with your friends in the hot jacuzi which you have intentionally deleted months ago - but someone from your team tweeted the direct link to it.
After the recent bug fix, these links would only have a self life for maximum 30 days, and in some cases they will die even faster. That's still a little shady, but better than that old picture of you bonging a beer at a college tailgate living somewhere in the labyrinth of Facebook's servers for all eternity.