You can't make me use the Delicious thing! I'm feeling very naughty and defiant here but am going to skip this lesson (I can always revisit at a later date, can't I?). I did set up an account when doing Library 2.0 and put a whole load of sites on there which I then deleted them from my Favourites - and haven't used them since.
As with Twitter I can see they are potentially really useful, especially for RA or Reference work, but you'd need to get a large no. of staff on board so one person is not the only one doing all the work and our staff -especially the branch staff who are the ones who know best what customers want - just don't have the time.