Basically, it was too hard to use. :-) I had to post very simple, step-by-step instructions as a comment to walk them through it, even down to the "click on the link and hold, then drag the mouse to the toolbar and let go" level. Several of them also didn't realize you needed to be on facebook.com to use it -- they tried just clicking the bookmarklet link on the original page.
The kind of people who can most benefit from a tool like this -- i.e., not the elite geeks who probably already have awareness of FB's privacy issues and how to address them -- need extremely straightforward, "don't-make-me-think" instructions to gain value from it.
These people aren't stupid -- they just don't have the same intelligences we have or assume others to have.