If you enter youtube.com into Site Explorer, you will find that stats will take a long time to load.
and if you click on the Organic Keywords report, you will find the "Couldn't fetch data" sign show up, with a red error message:
The same goes for Amazon:
You will find that this also happens to similar sites that are large, and have hundreds of thousands of pages.
This is because we do not show keywords for websites with more than 100 million keywords per country due to the amount of data required.
If you would still like to perform research on Amazon, YouTube or other similar sites, try using a specific URL instead.
Here's an example (I'm using the Amazon URL of the Art of SEO book):
Same goes for YouTube (I'm using one of our SEO tutorials):