Although Google Analytics has been in the market for around 2 years, it has captured one third of the world’s top 500 websites.
Pingdom (also using Google Analytics) had put Alexa global top 500 sites under observation. They found 161 out of 500 top sites using Google Analytics which is 32.2%.
This application was just another statistics package on market until Google bought Urchin (on which this application is based on). Google actually took a commercial service, rebranded it and produced it for free (that is why it is so popular).
Due to its competitive and cost free nature along with the strong brand, it has dominated the market in a very short span (and still growing).
For Google it might be a secondary statistics website but it has succeeded in capturing the market effectively. This shows that whatever they do, they do it professionally.