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:

// USER-EDITABLE VARIABLES // enter up to 3 domains, separated by a space var sites = ['del.icio.us digg.com ']; var opts = { width: 380, // width in pixels (max 400) height: 300, // height in pixels (max 300) type: 'r', // "r" Reach, "n" Rank, "p" Page Views range: '6m', // "7d", "1m", "3m", "6m", "1y", "3y", "5y", "max" bgcolor: 'e6f3fc' // hex value without "#" char (usually "e6f3fc") }; // END USER-EDITABLE VARIABLES AGraphManager.add( new AGraph(sites, opts) );

//

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

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