Twitter and Facebook make you MORE productive! according to this article in WIRED
…for knowledge workers charged with transforming ideas into products — whether gadgets, code, or even Wired articles — goofing off isn’t the enemy. In fact, regularly stepping back from the project at hand can be essential to success. And social networks are particularly well suited to stoking the creative mind.
The article talks about aiding creativity, but I would think that especially with Twitter, among librarian and techie users there is a huge amount of information and link sharing that goes on that is of enormous benefit for keeping up to date with tools and technologies. You just have to Twitter follow the right people.