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.