Facebook took it down. These other forums have not. Please don't pretend that this is a one off event. These forums are home to all kinds of needless, immoral content. Child porn, people being murdered, etc, etc. It is an active recruitment ground for fascists and white supremacists.
The imageboards are ephemeral and probably expired before Facebook took the video down. Child exploitation is regularly reported and removed promptly from the imageboards.
One of these things is not like the other.