Facebook, I guess you could use it to disclose all sorts on personal information given their record regarding security settings. Libraries could use it to promote events etc. but then Twitter appears to be quicker and just as effective.
It was a bit disappointing to see everyone use their own profiles to complete the Facebook challenge, it removed some of the mystique. And is it just me or is someone having a laugh with the security verification codes. Some of them were rather suggestive.