"Monopoly" refers to their share of a particular market, not their share of how you personally watch video. I don't think Amazon's streaming numbers even compare and Bittorrent is not a commercial entity. iTunes is kind of a competitor, but their business model is so different that it's questionable whether they're really competing for the same people in general.
Amazon's numbers might not compare right now, but they are very well positioned to elbow their way into Netflix's market.
I found an estimate[1] that Amazon has 121-131 million customers, of which about 5 million are Prime members. Those prime members pay less per year than a Netflix streaming customer, and they pay it mostly to get free 2-day shipping, so they're already deriving a lot of value from it even without the streaming. Amazon just dropped a big library (although not as big as netflix's...yet) of free streaming videos in their laps. Word will spread, plus they get to advertise this new service essentially for free to their other 120M+ customers. They also have tons of servers to handle all of this.
Amazon could also elbow in on netflix's DVD rental service pretty easily. They already have a worldwide rapid-shipping system in place, including handling plenty of returns. I'm sure they could add DVD rentals with two-day shipping each way if they wanted to. I'm guessing they don't want to, because they don't see a lot of profit in it, and maybe also because it could hurt their already-established business of selling DVDs.
Here's the question: of those 5M Prime members, have many are prime Prime and how many are through the friends and relatives? I'm an Amazon Prime member through a friend (i.e. I didn't pay the $75, I think a Prime member can have up to four people use Prime services) and Amazon doesn't let me stream videos.
I honestly don't know. It might also include people that get Prime for free (new moms, college students, etc.), who also don't get streaming video. Assuming that the 5M figure is correct, and that it includes everyone with access to Prime shipping, it might be as little as 1M or less who qualify for the free streaming, although my gut tells me it's probably a lot more than that.
All that being said, I don't think it's all that important. I wasn't trying to imply that 5M current Prime members compare to 24M Netflix streaming customers; I was trying to say that they have a really big foot in the door, which they could use to rapidly leverage their way into the market. The more important number are the 120M+ customers. Amazon can pitch them with, "You didn't want Prime for shipping, but maybe you'd like it for videos! (Especially if you're dissatisfied with Netflix!)" There are also rumors that they will soon let Prime members "rent" eBooks on their kindles for free (with a limit on the number of titles they can have at any given time). That would pull more people into Prime, and some of those people would inevitably take advantage of the free streaming.
