It's worth noting that the freedom of expression is generally believed to apply more broadly than just to governments.
For example, from the UN Declaration of Universal Human Rights, Article 19:
Everyone has the right to freedom of opinion and expression; this right includes freedom to hold opinions without interference and to seek, receive and impart information and ideas through any media and regardless of frontiers.
It's my opinion that Facebook is impeding on that right, and that it's going to come back to bite them at some point.