I've tried to keep myself separate. Facebook for friends, Twitter for professional stuff. There has been a bit of a crossover, but I try and stick to that rule.
However, there are some interesting groups on facebook that I'm a member of. National Trust Libraries group is one for discovering the treasures often hidden away in those grand houses. Library Manifesto makes for fascinating, if somewhat depressing, reading on the effect and future effects of government cuts, and there are loads of support groups for libraries that are threatened, like Cambridge's Arbury Court . And to conter-act that, the ever cheerful Orkney Library.
I'm still finding twitter confusing. I've heard rumours that I should follow a hash tag. I know the UK is full of Olympic fever, and that the torch does look a bit like a carrot from Camberwell, but... ! Anyway, I am yet to find a hash tag that isn't something to go with Justin Bieber fever, One Direction, or some so-called comedy phrase. Any suggestions?