I think it is a fantastic idea that libraries have started to use social networking services to reach patrons (mostly teens).
Firstly, its more appealing for the teenagers as they think it's "cool" to use social networking, and secondly because it's much easier and quicker for the libraries to reach these patrons through social networks than any other method (such as mails, e-mails, phone calls etc).
Reading some articles, there has been positive and negative views on libraries using social networking, however I think as long as the library uses it efficiently, and make the whole thing more approachable to patrons, than it will be a huge success.
Plus, I totally agree with what has been mentioned on http://www.libraryjournal.com/article/CA6375465.html about teenagers not receiving e-mails, as most of them contact each other via social networking (leaving a message on their blogs, etc).
As long as the libraries use social networking as a two way connection, then it's perfectly fine. Through social networking, the libraries can introduce new ideas, updates on library news, and information on patron's account etc.