Using labels= in the bookmarks macro without specifying a Confluence space renders to an empty string. According to the socialbookmarking-plugin documentation, if a space isn't specified it should default to the space of the current page. I'm attaching a patch that makes the behavior match the documentation, so e.g. {bookmarks:labels=todo} lists all the todo bookmarks in the current space. All of the logic was already in place, but a misplaced check for a null space kept it from working.