I can definitely see the advantages of using a social bookmarking site, especially say, if you're a PTH and have a set of websites that you would like to access and use often at the various branches that you work, or like someone else mentioned, if you work Sunday's at a branch other than your 'home' branch, you could use it then, as well. Being able to access them from any computer would make things much easier, I think. I haven't done too much with my own yet--between work and school, I'm lucky to have found any time at all to even read through this Web2.0 stuff at all right now!
Tagging is something that I can see pro's and con's to--not everyone will use the same words to 'tag' so there won't really be that much consistency. What would make perfect sense to one person could be Greek to someone else. I think for personal use, say in LibraryThing it would be helpful--if you're doing your own tagging and using it yourself, you'd know (presumably) what you're talking about!
Yippee.....I think I'm almost done!!