Just to give you an overview on the social features of SharePoint Server 2010, it consists of the following social features:
• Note Board
• Public tags
By default, all authenticated users can add social tags to documents and other SharePoint items & its visible to everyone.
• Private tags
A user who adds a tag to a Web page can indicate that the tag is private. Other people cannot see the fact that the tag was added to the Web page. Other people do not see the tag in the user’s tag cloud, unless the user who added the tag also applied the same tag to another Web page without making the tag private.
• The Note Board helps users express thoughts in their immediate context rather than having to move to e-mail, instant messaging, or phone. For example, users can make comments about a Web page while they are viewing the page. Other users can then see the comment and a link to the Web page, which they can visit if they are interested in the subject. This immediacy helps My Sites and My Profile pages become centralized places to manage public conversations.
• SharePoint Server displays the Notes, 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 Notes.
• 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, Notes that relate to that Web page will not be displayed for others.
In our case, Notes are visible to others for the non-PDF attachments(i.e., Excel, Word-doc..) as PDF files/items are getting not crawled in our environment