I use Twitter for its accessibility and usability to post a snapshop of my day in a short time frame, and its medium to promote our library service. I am developing my objectives in the use of social networking in the library service, but have found some positive examples of libraries using twitter to promote new books, to alert people to closures or program times, etc. Twitter's ability to gather an audience should be utilised to personalise the library's blog. Post positive messages and show your wares, and potential clients and industry members will come to understand your direction. It's a great medium to show what you value. Love visiting authors? Twitter about those who've been and who else you have programmed in. Specialise in rare medical texts? Twitter about special finds and applications. Refurbishing your school library with government money? Twitter about the effects and student benefits. And you can twitter from your phone - even more portability.
I've found bands do well on MySpace, check Bridezilla. And Operator Please, discovered via their MySpace weren't they? - are in both places. It's a place of promotion, so you'd have to utilise it in a library somehow. I've seen too many media articles recently using someone's Facebook site as a source of information, like their relationship status or whatever. Certain magazines could do a double spread derived from that status inf. But then, I've found a more sophisticated Facebook site: Deakin. I have a lot of exploring to do. And yes, I found here that Boroondarra and Deakin were two early adopters of Facebook.
Boomerang Books is on Facebook, with author William Kostakis as moderator.
Use FB to keep current on conference discussions: IFLA 2010.
I like Facebook over MySpace aesthetically. The page looks cleaner. Lots more exploration required to determine applicability and ability for outreach.