I don't make any attempt to separate the professional and personal sides of my life. I take the view that as a whole person, all the parts are the sum of the whole, and I would find it difficult to keep them completely separate. Unless you do succeed in keeping the different parts of life firmly in their boxes, one effect of social media is to blur the boundaries. Colleagues have no doubt recently discovered things about me they might not have guessed, and I've certainly had new light shed on the working lives of some friends and family which I would not have known much about before. I try to behave online in the same way I do in real life, which mainly means not swearing in public, and not necessarily sharing political and religious opinions with all comers (but I do have some and they are not all secret!)
As one whose career began in the days when The Librarian was a distant figure who swept past you in the corridor without apparently knowing your name, I rather like the "levelling" aspect of social media: some senior figures in the profession engage readily with those at the beginning of their careers. That's not without its dangers, particularly within an institution: imagine having to sack someone who was a Facebook friend, or a disciplinary issue with someone you have been chatting to on Twitter! Could social media distort a recruitment process? Make difficult decisions at work more difficult? I'm not in a position of having to supervise staff much now (been there!), but I can see the potential pitfalls.
My Facebook page is fairly bland: I took the opportunity of the recent profile changes to remove educational details, &c., because I was getting some pretty silly targeted advertising, but it wouldn't be hard to work out where I've been and what I've done. For sentimental reasons, I'm clinging to my film camera - ah, those far-off days twiddling things in liquid and the magic of watching the picture slowly appearing on the film! I know this can't go on forever, I admire the photos which others share, but as I haven't yet succumbed to the digital age there are not many images of me out there. I suspect that a lot of the sensitivity around about Facebook is about photographs.
The Welsh-speaking world is quite small: we brush up against each other in more than one sphere and the idea of completely separating work and the rest of life is even more difficult among the Cymry Cymraeg, especially in smaller places - I bet anyone from Aberystwyth could tell you the religious affiliation of lots of people they know professionally, for instance. (Interestingly, sometimes this world does exist in a separate dimension: you might have a Welsh-speaking neighbour or colleague and never realise that within that Welsh world your neighbour is a famous poet or singer!) Other things can undermine any hope of separating life at work and life outside, too. If you have children, you are thrown together with other people who have children the same age (who could be colleagues, senior or junior, academic staff, or students). My comments refer to academic libraries, as that's where I am now, but for anyone working in a public library and living in the community they serve the boundary can be more of an issue, even without the additional social media factor.
Privacy is maintained by a certain amount of self-censorship, and I'm lucky enough to work somewhere where secrecy is not necessary. On the whole, I think I'm happy to stick with my mixed approach, while bearing in mind the things that might derail it.