A few years back the civil service had it's own internal twitter clone. It's goal was for Whitehall types to be able to network but in practice it ended up colonised by benefits call centre staff who were stuck at thier pcs and often had time to kill when call volumes lulled. It was some of the most fun I've had at work with memes, running gags, songs of the day, twisted lyrics, mornington crescent, a proper, successful internet community of professional people from accross the UK, largely like minded, legitimately goofing off in down time. It didn't affect my productivity ( I was certainly happier in my crappy job), we kept it clean and within obvious bounds of proper behviour and although we were frowned upon (Freedom of information requests would have shown that this work tool was just social funtime), they couldn't just close us down.
It did get killed in the end after I'd left the service but I'm good friends with many of the people I met there on other social net works.
NOw if you'll excuse me I've the llamas to clean out. #Jelly_baby anyone?