Lots of libraries are getting on Twitter. But I doubt many of them are using Twitter in the way this librarian suggests.
To sum up the blog post linked above: the writer set up a series of Twitter searches for words like "cite", "citation", and "need" AND "book", geographically limited to a relatively small radius around his library. This allows him to monitor the stream for tweets that describe needs that his library could answer without actually containing a reference to the library within the tweet. He can then respond to tweets that express a need that the library could fill.
I think that this is a really, really smart idea.
I am aware of libraries that monitor Twitter for mentions of themselves. This allows them to keep track of what people are saying about them, and to respond when necessary. But the only people who are going to be tweeting about the library are those who already know about it. An important question to ask for any marketing effort is how we reach not only the people who already are aware of us and use our services, but how we reach our nonusers, the people for whom we are not the logical place to go for information. An intelligent set of searches on Twitter, geographically limited to the library's service area, is a great example of how we can use our presence on Twitter to reach out to nonusers as well as users.
This idea then begs the question: What other social networking tools are libraries using that can facilitate ambient awareness? How can those tools be used to discover the needs of nonusers as well as users, and how can they help us fill those needs?