Force.com sites provides out-of-the-box integration with Google Analytics. Use Google Analytics to track the usage of your sites and site pages, including number of visits, number of page views, average time spent on site, and more.

After setting up a Google Analytics account, copy the "Web Property ID" to the "Analytics Tracking Code" field on the Site Edit page for the site you want to track.

To track the Visualforce pages associated with your site, enter the following tag in the site template for those pages, or in the individual pages themselves:

You can also reference the Analytics Tacking Code using the {!$Site.AnalyticsTrackingCode} expression in Visualforce pages and email templates.