What's Wrong With Alexa?

Why have many "web 2.0" service seen a significant dropoff in traffic in alexa in the past six months?

I used to find Alexa valuable for at least relative numbers. But given what’s been going on recently, I am not sure it can be trusted for anything.

Check out this chart for delicious and digg:

These two charts are almost identical in their shape. How can that be?

Here’s comScore’s data on digg and delicious: