We just added the ability to track your Site Builder site with Google Analytics. To enable Google Analytics, you'll first need an account, which you can get here.
You will receive a code, as seen in the image. Go to Site Builder and click "Edit Site Information" under the Administration section. You will see the field available to place your newly minted code. Save the information and republish your site when prompted. That's it!
A couple of warnings about Google Analytics. First, we don't support it in any way. We only facilitate you being able to use them, and do not offer any training on running reports within Google's system. Second, Google Analytics is not set up to track file downloads. We are looking at ways to integrate file tracking, but right now you will only be able to view statistics regarding page views and other user data acquired by Google.