You beat me to it Nick. I was going to say visitors & conversions too
Alexa is pap - it only counts people who have the alexa toolbar installed, so if you wanted to & you had say a very big corporation, you could get all computers fitted with the alexa toolbar & get everyone to visit the website every day & skew the results.
You could do the same thing with a popular site by encouraging visitors to install the toolbar. The BNP is apparently on around 31000 most popular website in the World, which is surprising since they got about 1 million votes only.
They outrank Labour, Conservatives, and all other UK parties - so clearly alexa is poop.
Page rank doesn't really help you much either unless you're after kudos only.