The photos made their way around the world to Zuck-everlasting fans after members on an online bodybuilding forum noticed "a glitch in the system used to report inappropriate images on Facebook," according to MSNBC. This actually allowed anyone to view anyone else's private photos, not just the Zuck's.
Facebook has fixed the bug, it says:
Earlier today, we discovered a bug in one of our reporting flows that allows people to report multiple instances of inappropriate content simultaneously. The bug allowed anyone to view a limited number of another user's most recently uploaded photos irrespective of the privacy settings for these photos. This was the result of one of our recent code pushes and was live for a limited period of time. Upon discovering the bug, we immediately disabled the system, and will only return functionality once we can confirm the bug has been fixed.
The privacy of our user's data is a top priority for us, and we invest significant resources in protecting our site and the people who use it. We hire the most qualified and highly-skilled engineers and security professionals at Facebook, and with the recent launch of our Security Bug Bounty Program (http://www.facebook.com/whitehat/ ), we continue to work with the industry to identify and resolve legitimate threats to help us keep the site safe and secure for everyone.