On public-facing websites enable the Usage Cookie to track anonymous visitors.
SharePoint 2013 contains new web analytics capabilities that you can use to display relevant content to your users. The new web analytics are based on SharePoint 2013 Search which, in order to work correctly, need to distinguish different users. For authenticated users this isn’t a problem, since each user has her own personal account. On a public-facing website however, where the majority of the traffic comes from anonymous users, there is no identity to use to distinguish one user from another. SharePoint 2013 allows you to track anonymous visitors using the Usage Cookie that can be enabled per Web Application.
Originally posted at http://blog.mastykarz.nl/wcm-tip-11-usage-cookie-track-anonymous-visitors/