Businesses seem to have embraced Twitter. The Dell Outlet has a Twitter presence. Become Dells' friend and you'll get limited time offers on refurbished equipment.
Is the concept applicable to libraries? I'm not sure. We don't want to restrict "this offer only to those that can see it" but maybe it could be a way to promote new books. "Just in - -- Bestseller, reserve it now" I'd probably friend my library if they did something like that, but is it worth their time and resources?