Because that’s too obvious and also Facebook’s userbase’s ideologies lean the opposite direction from the establishment, and Facebook is their best tool for sharing information that happens to run counter to what the establishment (media, government, etc) finds to be ideal.

Besides, there are only a few sites that most people ever visit. The internet is “small” now. Tackle the biggest threat, the rest will fall in line - likely with glee since they already enforce pro-establishment censorship policies as it is. Twitter, for example.