If you (publicly) tag items, the tags will show up in the tag Cloud; however it could happen that when you click on the the tag no result will de displayed; one of the reason is that the Tag security trimming; indeed, according to the sdk:
adding a tag, a note, or a rating to a Web page creates an activity. Before SharePoint Server displays an activity, it uses a component called the security trimmer to determine whether the current user has permission to view the Web page that the activity applies to. If the user is not permitted to view the Web page, SharePoint Server does not display the activity.
As the search service crawls Web pages, it records the permissions that are required to view each Web page. The security trimmer uses this information to determine whether a given user has permission to view a specific Web page. If the security trimmer has insufficient information to determine whether a user has permission to view a Web page, it errs on the side of caution and reports that the user does not have permission to view the Web page. As a result, if the search service has not crawled a Web page, activities that relate to that Web page will not be displayed.