MTagger, a social bookmarking tool, launched in the winter of 2008. MTagger allows users to tag a webpage on the library site or anywhere, and catalog records, or digital images. The tool was intended to enhance findability across collections and to expose ?hidden? collections. After launch, the service did not meet our expectations for use, so we embarked on usability testing. This talk covers the questions we asked, why we asked them, and how we're responding.