Alexa's numbers are crap. Total crap. They only reflect your traffic in the loosest way. There was a period of time where our traffic doubled and the alexa actually declined. It eventually catches up though....think of a random distribution around the true value.

Just so you don't get the idea I'm picking on Alexa, estimating this stuff is hard (or the people doing it are incompetent, but that seems unlikely with so many entrants). Quantcast and Compete.com are even worse, and ComScore is expensive but no better than Alexa.

I've always found quantcast to be much more accurate, although their number for justin.tv looks low: http://www.quantcast.com/justin.tv