The social tagging capability in SharePoint 2010 takes several dependencies on different service applications: the Managed Metadata Service application (MMS) for storing and providing tag definitions, the User Profile Service application (UPA) for storing the actual tagging and notes, and the Search Service for several tag-based web-parts to show tagging information such as those on the tag profile page. The identity used to run your SharePoint 2010 solutions and the content crawler identity needs to have access to the service applications that manage and store your social tagging data.
I recently added a new web-application to my development farm, and today to my surprise I noticed that the "I Like It" button in the global area was missing. In addition, the button was disabled in the item "Tags and Notes" ribbon.
The Tags & Notes buttons worked as expected in other sites in the farm, so the "Social Tags and Note Board Ribbon Controls" farm feature was activated, enabling these buttons for all sites in the farm.
I first verified that I had the "Use Social Feaures" permissions in the User Profile Service application, and did some tagging in another SharePoint site in the farm, ruling out that this had something to do with my SharePoint user. So I knew it had to be something related to my newly added web-application identity, having had problems with the search-driven tag cloud and tag profile pages in the past.
The solution is to grant the identity running the SharePoint web-applicaiton the correct connection permissions to the two service applications. My new web-application lacked permissions on the Managed Metadata Service application. Go to "Central Administration > Manage Service Applications", select the MMS application row in the list, and click "Permissions" in the ribbon to configure the connection permissions.
Add the managed account used as the service account for the web-application, and assign it permissions to read and add new terms to the MMS term store. Write access is required to allow users to apply tags never used before. Full access is not required. This will show and enable the "I Like It" button and the "Tags" tab in the popup dialog for "Tags & Notes" as expected.
Revoking the service application connection permissions for a web-application identity can be used to remove the social tagging and note board tools from contained sites. However, while the "I Like It" button is hidden when revoking access to MMS, the "Tags & Notes" button is not hidden when revoking access to UPA, rather a non-functional popup dialog is shown - not the best user experience. The following figure shows the effect of having access to MMS but not to UPA:
So, if there is something strange with the social tagging and related web-parts in your SharePoint 2010 solution, always make sure that web-applications identities and content crawler identity have applicable MMS and UPA service application connection permissions.
If you use the Note Board web-part, and it suddenly stopped working by not showing any existing notes and you cannot even add new ones - and only the web-part title is shown, then make sure that your Search Service Application (SSA) indexing is crawling properly. This can happen after applying e.g. a cumulative update. Remember that you can always check if the tags and notes database is operational using the UPA.