I'm not familiar with this aspect of Facebook, but it looks as though they do provide a reason in the message, though: "...includes a link that our security systems detected to be unsafe." unless this is just their catch-all phrasing for links that they don't like for some reason. Have you looked into why they may think your site is unsafe? Is your user generated content properly sanitized? Are other *.fm links working? How are you hosting (maybe the IP range is banned by some other malicious activity)?