To provide even more context, let's assume that we're might want to use the social bookmarking within an institution in the following ways (this list is not necessarily complete):
By registering all the students on a particular course to the system so they can use it in course related exercises;
By registering every student who joins the institution so they can use it as a resource throughout their registration period, and even beyond (i.e. when they graduate);
To allow the system to integrate with pre-existing authentication systems;
To allow students to share knowledge between cohorts (e.g. so one year's coohrt can leave a legacy to the next cohort);
To provide an environment where student project teams or tutor groups can share bookmarks and comments privately amongst themselves;
To provide a bookmarking component in an e-portfolio system;
To maintain a needs-serving link with graduates/alumni by continuing to host their bookmarks (perhaps supporting this service by advertising relevant courses or fundraising initiatives to such alumni at a future date;-);