Philosopher's Nightmare
Any time you're witnessing a conversation spiral downwards and are able to identify the rhetorical techniques participants are fueling the descent with little hope of being able to recover from. Techniques include cherry-picking, generalizing, the moral high ground, appealing to fear or popularity, aggressive (including passive) behavior, explicit bias and persistent rambling with an intent to win over the other side. You're confident any contribution or attempt to help move the conversation forward will be ignored or met with immediate resistance and rejection.
That post on Facebook about gun control became a Philosopher's Nightmare with most of the participants attacking and distracting each other.