Ars Technica first wrote about this story a year ago and the problem hasn't really gone away so they've written about it again. At issue: what happens to your Facebook photos when you delete them. Or rather when you think you've deleted them because you actually haven't. Back up copies of the photos are stored all the time on Facebook servers and while the pictures might not show up on your page, the direct links to them still quite often take you to the picture itself. So that incriminating party shot you wanted to make go away might still be there.
Facebook does acknowledge now that the photos are technically accessible by some people, but basically repeats the line about working with the CDN. "It's possible that someone who previously had access to a photo and saved the direct URL from our content delivery network partner could still access the photo," Axten said. "However, again, the person would have to know the URL, and the photo only exists in the CDN's cache for a limited amount of time. We're working with the CDN to reduce the amount of time that the photo remains in its cache."