How do websites like yahoo and google track their market share?

[Limelight]

I see sites showing chart population on market share on sites like google, yahoo, youtube, ebay ect. I was wondering how do they track it?

[stefaan_freddie]

there are several tools. For example: you have the website alexa.com. That site provide a tool for firefox where you can see the rank of a website. And the tool registers also the pages you visit and send that info to alexa.

this is the top (global) 10 at the moment on alexa.com: (you can also check for each country seperately)

1. yahoo
2. google
3. youtube
4. windows live
5. msn
6. myspace
7. facebook
8. hi5
9. wikipedia
10. orkut

see http://www.alexa.com for more info