If restaurants can use Twitter to tempt diners
(according to this Reuters news item: http://www.reuters.com/article/lifestyleMolt/idUSTRE53I11Z20090420 ),
why not libraries?
A quote from the story gives a warning for already over-worked library staff who are considering adding this duty to their list:

"It does take time and effort and care and feeding," said Chris Brogan, president of New Marketing Labs, a new media marketing company. "If you go silent it shows. It's like not answering the phone."