I think social software can be used by libraries in a number of ways.
The blog is an excellent medium for organizing information on particular issues. Libraries often have a useful links page which has a simple list of sites where information can be found, but as new and interesting websites keep getting added to the internet daily this kind of list is not as useful anymore. Blogs can help organize the links into specific categories of interest. RSS feeds keep the content up to date and the posts and comments allow for interaction. Blogs are also great for libraries to use as electronic message boards.
Social tagging is a great way for libraries to improve internet searching and assist in finding information for library users. It can also serve like blogs as a means of organizing links to website into various categories for library users.
Instant messaging has a place in the library and can add to library services. It is very useful for pushing links to library users and to provide library users with an instant connection which leaving an e-mail request can't do.
Podcasting could be used to give voice to the library and to communicate in spoken conversation with library users on the internet. This could enhance library services.
Wikis can be a collaberative tool for libraries, but except for working on a resaerch project with a team or other internal team projects, I don't see the wiki as being that applicable to library services. I also don't see social networks contributing much to library services. I think they are useful for connecting to the greater library community and for library users to connect to each other.
Those are my initial thoughts. Will post more later