Google+ platform websites integration using the Google+ platform tag.
See what your Facebook friends have liked, shared, or commented on across the Web. This includes all variations of Like, Share and Follow Buttons, Embedded Posts, Comments, Activity Feed, Recommendations Feed, Recommendations Bar, Like Box and Facepile.
Sites who has this tag are verfied by google to use WebMaster tools
A canonical link element is an HTML element that helps webmasters prevent duplicate content issues by specifying the "canonical", or "preferred".