The best example I found of libraries using social networking in a creative way is the National Library of New Zealand's page on MySpace:
The creation of this MySpace page is a novel attempt by the National Library to raise awareness among musicians of New Zealand's legal deposit obligations and the provision of quality cataloguing and storage for their work. This is a genuine and valuable example of how social networking sites can be used in a positive and practical way.
However, I felt that most of the other MySpace pages that libraries have set up are vapid and trivial and just seem to be trying too hard. To really engage with young people we need to offer some real content. Library users are quick to distinguish real value from surface gloss, however trendily it is dressed up.