I do not consider Nazi ideology an opinion. Their ideas are purely based on maintaining a power dynamic. There's no logic or legitimacy to their ideas, and thus they are not opinions. They change the ideology as needed to maintain power and promote their own strength.

For a historical viewpoint, simply look at how often the German Nazi party would change what classifies as being a Jew or "undesirable". It was only ever about putting fear into people and maintaining control. They had no real opinions on why these people were undesirable - those could be made up after the fact.

If Nazism was an opinion, I would be willing to defend it. But it's not, and it thrives when it's given the legitimacy as such.

EDIT: To try and tie this back to the topic at hand, are blatant lies still "opinions"? What about death threats and hate speech? My point is simply that we already make distinctions about what kind of speech is free, so I feel that Facebook instituting policies that align with these existing rules makes sense - even though I feel that they are becoming too large to be the de-facto source for information on the Internet.