Now this actually raises an interesting question. Would you define a monopoly based on the service a platform offers, or the resulting combined services created

Now this actually raises an interesting question. Would you define a monopoly based on the service a platform offers, or the resulting combined services created by everyone utilizing the platform? One could argue that any service that allows you to upload and share video is a replacement for Youtube from the perspective of the platform's service. But you're auguring that unless another platform has a comparable amount of content, it is not a viable replacement.