I started to look into Alexa a bit today because it is being used by some advertisers as a benchmark for websites. Funny thing is that when i looked into it I found that the ranking itself is so easily manipulated by a site owner that I can't see why anyone would ut any credence into their ranks. Just take a look at this simple Google search I did and see for yourself:
An enormous amount of information comes up and is readily available. Software use to manipulate it, tricks to manipulate it, and anything else that people have figured out to crack the system. Doesn't sound very reliable to me. The Top 10 is legit I'm sure, but the Top 5oK? I don't have any confidence that is the case.