Favs tries to extract tags from the different sources it is able to access. To get an overview you can open the Most Popular Tags cloud view from the app menu (Window > Tags or press CMD+2) that gives you an overview over the 75 most used tags. By dragging in the cloud you can change the focus and this way explore your tags. If you click in a tag it will be used as a search term in the main window and you'll see all related articles.