I get asked quite frequently why I like Twitter, while Facebook leaves me cold. Easy. It's short. Doesn't take much time. I don't have to interact, be friends, respond, or do anything at all if I don't want to. Like most Nascar fans now, I follow fav drivers (Brad Keselowski is a Twittering rock star), and people who "get" Twitter, like DeLana Harvick and John Daly.
As a writer, I check in with Publisher's Weekly, the Library of Congress, and assorted editors and publishers. Most provide links to the most up to date news and interesting blogs and articles about the biz. I learn stuff that could take months to filter down the food chain to the lowly author. For example, right now something odd is happening with Avalon Books. (Thomas Bouregy & Co.) If I wished, I could Tweet about it and alert any writers following me to keep their manuscripts in their drawers until the real story comes out. Unless you're on the private list for Avalon authors, you wouldn't know. Twitter opens up the dissemination of information quickly and concisely. And I can control who follows me easily.
So I guess the short answer is, Twitter is fast, easy, and my kind of shorthand. 140 characters, that I can handle.