1. It requires no effort -- none -- on the part of most users. They literally have to just press the play button to watch a video. There's no logging in, no learning strange conventions like Twitter hashtags, no confusion about the purpose of having a Facebook "wall". Just a pure and simple purpose: Watch videos. Absolutely anyone can figure out how to use it.

2. It has become the default forum for hosting video. For those who wish to upload videos to the Internet, there's no question about where to go: YouTube is the default location. Anyone who bothers trying to come up with an alternative architecture would have a massive degree of effort required to convince people otherwise. Unlike Facebook, which took over social networking dominance from MySpace because MySpace ceased to have any "cool" appeal, there's nothing cool or trendy about using YouTube. It's just there, and doesn't attempt to make any waves of its own.

3. The amount of investment by users in YouTube makes it hugely valuable. Posting videos there -- many of which are not archived anywhere else -- is like putting something in one's safe-deposit box at the bank. Users have so much invested in their uploads that it would take a massive force to cause them to take them down and put them anywhere else.