I've never gotten reliable results from compete, whenever I look at the results for a website for which I know the real statistics they are always completely wrong...
For example, one of the website I manage has 10 times more unique visitors than compete reports. Another website has been growing in the past few months but is shown as declining in the past few months on compete.. So I don't know what they are measuring but as far as I'm concerned it's about as efficient as using a random number generator to determine the number of page views of a website..
http://blog.stackoverflow.com/2011/04/stack-exchange-traffic...