If you want to go after the Libyan National Army for doing bad things, you have to go to war with Libya and defeat their army.
The trouble with going to war with Libya if you are the United States is the Atlantic Ocean and the absence of a real driving interest such as self defense that would lead Americans to support a war with Libya.
In order to goad Americans into a war with Libya, the best hope for doing so is to wave bloody shirts at them and somehow convince them that Libyans are going to blow them up at any time now unless we go to war with Libya ASAP.
Facebook and the rest of them don't have a duty to simultaneously prevent the wrong people from viewing Ogrish-style execution videos while also preserving them.
So they want Americans to be radicalized in favor of war against Syria and Libya (causing far more deaths than the typical radicalization which just results in teens posting lots of pepe memes), but not radicalized in favor of Islamic terrorists. That is just an impossible propaganda requirement that cannot be met.