I really don't use link sharing with students so I don't know which service would be best. I teach in a program for gifted kids, they only come one day a week so 'free surfing' is kept to a minimum--waste of time. Of course they do it they are working on a specific topic--but if we are doing a major group project I spend a few hours scanning for the best sites.
I used backflip for years, it has folders which makes finding stuff easier that delicious--I've gotten to the point where I just put the links I want on the curriculum websites--see example here http://connections.smsd.org/veterans
I also have a site called curriculum links http://connections.smsd.org/curriculumlinks/--I use it if I plan not to publish a whole website on a topic.