The first suggestion that comes to mind is to automate as much of the process as possible. I rarely use social bookmarking sites because, 1) they waste more time than they save, and 2) I don't have the patience to sort through the details of URL submission.

See if you can develop a heuristic to determine a user's favorite sites based on their browsing patterns. Convolve that into a set of browser extensions and expose the user data through a web front-end. Make personalized suggestions based on user history. Auto-generate site summaries by looking at page markup (size of fonts, page title), that sort of thing.