I can see a use for it. It seems to combine together a lot of different minds and that should at least give balance to an issue or inquiry if nothing else. The sheer amount of information can seem a bit daunting, but learning to organize and sift through the information is the job a librarian.
One thing that I don find a little bit strange about social networking sites, is that even when relaying "academic" information or information about library blogs, they reveal more of a personal nature than other types of information sites. For instance, when I did some looking at some of the popular tags, I was surprised at the amount of personal information that people revealed on their blogs. Does the informalness of the setting detract from the helpful information that it provides? Probably not, but it can be distracting. I find myself doing so much skimming looking for links when I visit sites like this. I don't know if I like that.