Google Trends for Websites is a new project of Google Labs that provides detailed traffic statistics based on searches made through the google search engine. It provides a service quite similar to that of Alexa, which has been heavily criticized by webmasters as being too inaccurate. Alexa bases its rankings on the statistics it gathers through its Alexa Toolbar, so if a visitor visits your site with the alexa toolbar installed only then will it be counted as a hit. This ofcourse makes it quite unaccurate. Google Trends for Websites however is based on provide rankings and traffic statistics based on searches and clicks made through the google search engine, which in my opinion makes it more of a reliable tool.

Here is an image of comparing traffics from Facebook and Myspace:

And the following is an image taken from Alexa:

Well, the overall shape of the two graphs seem to be almost the same but on closer examination you will see the difference. For example the point of intersection has quite a different timeline. Google Trends for Websites intersects the graphs between October 2007 and January 2008 whereas the Alexa graph intersects them at April 2008. So there is definitely a huge difference there. And its up to you to judge which one is correct.
Also, google does not show the daily amount of visitors to the sites. It is only good for comparing if you know the traffic for one of the sites. Say you know your daily traffic and you want to find the traffic for your competitor. Just plug in both the sites and use the difference in heights to estimate the amount of traffic received by your competitor. Anyways, I like this new tool from google and will be using it frequently in the future for my web projects. Lets see how popular it becomes!