While I do think the design is very well done, I'm not sure I see the benefit of something like this, vs. Delicious and other social bookmarking things.

Keeping it a gated community is going to make taking off VERY difficult, unless you have some over-sharing, highly prolific people in out of the gate (no pun intended).

While there may be groups that do want to keep their sharing semi-private, are there enough to build a meaningful audience (read: potential advertising revenue stream) for the service, or will you just charge for it (saw no mention of this, but may have missed it).