I used to use TGS excessively and TabsOutliner has completely changed my workflow. Now I just sort tabs into categories and then kill the entire window until I am in that context.
This service does look very similar to Delicious. Obviously Howl is displaying links per user, but the end result is the same.
You say that it isn't the focus of Facebook/Twitter and your right, but it doesn't need to be. Why not make your site plug into Facebook more? Allow people to like each post, or cross post all links onto Facebook. That way your integrating but still differentiating.
Good work on getting it all of the ground though, and good work on the domain name. Keep at it, something good will come from it.