Hi HN! I work for Facebook's Public Policy Team focusing on our Security efforts. Unfortunately, this was indeed a glitch in our spam prevention system.

We integrate with many different external blacklists (https://www.facebook.com/notes/facebook-security/link-shim-p...) including Norton and when there's a block on their service we provide a warning to our users. I have already reached out to Norton to notify them, however, the cause for the block is a question best suited for their team. We only use our external blacklist system to protect our users and would never use it to reduce traffic for competitive reasons.