Just visited a University of Omaha marketing class that was led by Bryan Jennewein, the social media director of Infogroup [http://www.infogroup.com/]. He had great tips for using social media to drive interest to your business. He spoke in general business terms, but I am going to insert the word "library":
-Transparency is the key. Include buttons/links to your library's Facebook, SMS text service, Twitter, YouTube, Flickr accounts on your homepage (and throughout your site, so include links on your catalog, your board page, and so on) - your customers will know right away that they can follow the library's activities using social media
-integrate your library's Facebook, Twitter, and YouTube account, leading viewers of one to the other, ultimately learning more about what you have to offer
-make sure that your social media accounts also include your website address
-talk "with" your customers, not just "at". Consider allowing comments from social media customers. Even the negative ones give your library an opportunity to directly address perceived (or real) lapses in service or resources, and you can "make good" to a customer with your other customers watching
-keep up with what is being said about your library by using Google Alerts [http://www.google.com/alerts], and/or the freemium version of AlterianSM2 [http://sm2.techrigy.com/main/signup.aspx]. (note from TH: possibly your library system has already signed up for the pro version of this service - check with the head office to see if they have, or are considering it.)
-address comments, good and bad, quickly and effectively. Have a plan in place on how response will be conducted/who will say it/what generally will be said, and then be ready to let your board/director/city administration know that it was handled.

Please let me know if your library has included social media in your general marketing efforts. I am collecting images of library homepages and Facebook pages. Here are the pages from the library where I work:
http://www.unmc.edu/library
http://www.facebook.com/pages/Omaha-NE/UNMC-McGoogan-Library-of-Medicine/188291528831