Hans Björkstam: My Social Bookmarking Soulmate

I have found him! My social bookmarking soulmate! Together, he and I will bookmark the digital world of coffee in a social manner! Ok, not really, but I have found someone with an interest in coffee who does a fair amount of social bookmarking himself.
Let me be frank and say that I understand the value of social bookmarking, but thus far have been unimpressed.  I'll concede that if you bookmark what you find interesting, tag it properly, and others do the same, you should be able to connect with users that share your interests and even find links that you wouldn't have otherwise.  However, in my experience, social bookmarking has never been that seamless.  Either I'm not tagging right or they're not tagging right.  Regardless, something isn't happening the way it should so the effect isn't ideal.  Hopefully Hans Björkstam can change that view for me.
I found Björkstam by running a simple search for people that have used coffee as a tag in the past.  Björkstam showed up as a hit on page two.  I decided to look more into him because he not only used coffee as a tag - he used espresso, beans, and brewer. Alright. I can get on board with this guy!
Björkstam is certainly an active bookmarker.  His most recent bookmark was 8 hours ago in fact (as of the writing of this post).  He bookmarks somewhere between five and ten items a week, and has been doing so since January 14, 2011.  (This just got kinda creepy.) Over the course of the last year or so, Björkstam has bookmarked 395 items.  The majority of them, 346 to be precise, are tagged coffee; 201 of them espresso; 107 - beans.  There are a couple things we know for certain - Björkstam has an interest in coffee and he definitely isn't new to social bookmarking.  But I have to asterisk the latter half of that last thought because while Björkstam may not be new to Diigo, I don't think he's really playing up the social aspect of the bookmarking site.  In the 397 days since he joined Diigo, Björkstam has accumulated exactly zero friends.  He neither follows nor participates in any public groups, and I don't think it's too much of a stretch to assume he doesn't contribute to any private ones.  In the 295 sites he's bookmarked since he joined Diigo, he has not added any comments.  Björkstam's Diigo seems more than anything to be for him - a place where he can bookmark his favorite sites without any social interaction with other people.  It's simply a catchall for his internet findings.
This, I find is a problem with social bookmarking websites.  Users are only interested in using it as a place to hold their bookmarks.  They're not interested in sharing them.  Have you ever used a social bookmarking site?  Did your purpose for the site match it's functionality?