Traffic figures are always finger in the air unless it’s your own site.
Compete used to be fairly OK for US figures but hopelessly wrong for UK based sites. The problem is that they’re like TV viewing figures – extrapolated (math term for guesswork) from small data.
Just checked and Quantcast gave me no results for a site I look after that has around 100k visitors/week. TrafficEstimate was out by a factor of 40 (it guessed 10k visits/month rather than over 400k)
Alexa is based on the people who voluntarily install their toolbar in a browser so position ranks are heavily skewed but are kind-of useful in comparing similarish sites in similarish niches.