The commodity that youtube sells is not only the ability to upload a video, it is the access to a network of viewers. It can be argued that due to youtube's network size relative to other video sharing website, it has a monopoly in that regard.
i don't agree. plenty of videos are being watched on facebook, instagram, twitter, etc, not to mention all the asian clones.
youtube just seems to be the most monetized platform for the time being, but i don't think it's anywhere near being monopoly in terms of access to users.