My point is that I don't have my own definition of words!
We should all use the common definition of words. That makes communication possible!
dictionary.com gives this definition: "exclusive control of a commodity or service in a particular market, or a control that makes possible the manipulation of prices."
That's... not what I expected. To me having a monopoly means being the only provider, but I'll bend the knee to the dictionary.
In this case though, with competitors at 10% or 25% market share, you can easily publish videos at a cheaper vendor.
When the entire pool of competitors is only 10%, it's not so easy. You can put a video up but nobody will see it. The network effect of viewers is one of the big reasons to want multiple major sites.
They're something like 75% of video hosting, and if you exclude vimeo for being paid-only then it's over 90%. That sounds like a monopoly to me.