Amazon owned Alexa changed the way it calculates their web site rankings.

Alexa started to rank sites in 1998. The statistics were derived from users of the Alexa Toolbar. Now Alexa integrates internet usage beyond Alexa Toolbar users. Alexa is now aggregating data from multiple sources to give you a better indication of website popularity among the entire population of Internet users.
Alexa had to make this move as I do not know anybody using the Alexa toolbar anymore and there are competing website ranking services that gain popularity like Quantcast (site).

Site ranking is one of the factors for advertisers to select sites for their campaigns. It is important that Alexa's statistics are reflecting a real picture, because many people just take the numbers for granted. The new ranking is good news for I4U NEWS as we jumped forward quite a bit. I4U.com ranks today at 5,572 in the United States, which is an improvement by about 2,000. The overall ranking jumped from about 25,000 to 16,460.
More details on the Alexa site.