My reaction to social bookmarking: eh.
I don't like the fact that I automatically ended up with another bar of "things" across the top of my browser (which I have hidden, along with Google's and everyone else's). Before we know it, we'll have so many of 'em that we'll be reduced to a screen height of 1" for viewing actual content or e-mails or whatever. My screen real estate is precious -- no, I don't have a low-res or teensy screen! -- and will only get more so as my peepers age.
Other than that, I really don't see the utility. User-defined tags mean missing connections to items tagged with synonyms. It's just a new version of the old controlled vocabulary vs. natural language debate.