Are your interests in social bookmarking (a la delicious/clipmarks..) or social annotation (diigo/fleck/stickis/trailfire ...) ?

The scene is pretty crowded and traction will be very difficult unless you have significant differentiator.

Based on my prior experience getting friends/family to use a social bookmarking service , most folks don't use any because organizing bookmarks never gets elevated to problem from a mere inconvenience. They have to shown thats it's really really easy before they become converts. Your trick is going to be how to reach this audience.