There're many movie theaters, but only one youtube in the whole world.

Youtube and facebook are natural monopolies due to their network effect.

All the older natural monopolies, water networks, electricity networks, landline phone networks, railroad networks, and the rest of them are either nationalized, or regulated heavily.

It's long overdue to add social networks to the list.

There's only one YouTube, but lots of video sharing sites. You can put your material on Bitchute, Vimeo, ....

Youtube's monopoly is not video hosting, there're many ways to do that.

Their monopoly is their social network. People want to publish video where the audience is, the global audience happen to be on youtube, and going to stay there because that's where most videos are.

Not absolutely. They shouldn't be able to show porn, for example, or some other destructive content. (And no, it's not a question of taste or opinion. That relativist claim is worthless and tired. We're not talking about vanilla or chocolate ice cream here.) I suppose in this particular case (and that matters: things like this can be particular) you would just criminalize those sorts of movies.

The idea that we shouldn't do such things because government could be used to impose horrible things is a bad one. Driving a car could kill you, but you wouldn't ban cars for that reason because cars are not bad in principle nor are they so dangerous that they should be banned. They are regulated, however, and this is good.