Social software can be used as a readers advisory tool. You can go where customers are, keep track of what you've read and connect with authors. Try several sites--they are mostly free and easy to drop if they don't work for you.
Hennepin County Library has a Web 2.0-style site called Bookspace. It lets you inform patrons when a book comes out, allows rss feeds & comments and the patron can create a profile. It was created with an in-house Coldfusion script.
Patrons can also create a weekly search in the Aquabrowser catalog.
Facebook can be used to promote the library's catalog and events. Authors have fan pages on Facebook to let readers know about visits & when new books will be coming out.
Libraries can create their own YouTube channel and upload fingerplays & booktalks. YouTube is also a source for author interviews and video tributes by fans to their favorite books.
Ning an easy way to create a group like mystery fans, science fiction, book club, etc.
Librarything is a site for cataloging one's books that is also social. The user can read other's reviews and get "reader's also like" recommendations--even books they would not like! Users create their own tags to identify books. Under the heading of "common knowledge" users add information like character names, movie adaptations and cover artists. Libraries can also publicize reading events there.
Goodreads is another social reading site. It offers book-swapping and quizzes.
Even Twitter can be useful--check out the hashtag #reading.
The Reader's Advisor Online blog is highly recommended by the presenters.
Handouts are available here.