I was fascinated by this piece on the Beeb about prospective employers demanding the login information from their job candidates. Can it ever be justified?
My reflexive response, is "hell, no!" but I think the real answer is a little more subtle. However, let's tackle the easy case straight away. If a prospective employer asks you for your username and password (as happened to Robert Collins in the Beeb article) then they are almost certainly making you violate the terms of service of your social media account (Facebook, G+, Twitter etc.) and you should tell them to take a running jump.
What if they just ask you for your social media account info so they can look at what you're posting publically? No harm there, right? Um, well...
There are very few positions where the social media postings of a prospective hire are relevant. You are under no contractual agreement with this company, so there are no non-disclosure agreements to violate. If they can Google your name and find your postings, fine; you are however under no obligation to assist them. (This is obviously a better cover for John Smith than it is for Thomas Wanker.) The only exception to this that I can think of is if you are under consideration for a PR-type role where your public social media persona is a significant issue. In this case it's only reasonable for the employer to ask you to point them at your posts, in case there's anything there that may blow up out of proportion if they hire you.
Perhaps you don't post anything of note publically? They may require you to give them access to your posts (in FB, by you "friend"ing them - in Google+ it's a lot more complex, since different posts are visible to different circles and they have no idea how many circles you have, or to which circles you post.) This is really treading on your toes, and even if you are being hired for a social role you should consider whether you regard it as a reasonable request.
In most cases, however, this request by a company's HR department is a very useful piece of information to a prospective hire. It tells you that the HR department is stuffed with humourless control freaks, and you should tell them to take a running jump. Alternatively, construct a new Facebook account; add a few very, very understanding friends, and make some suitably tasteful posts about Internet shock sites such as goatse.cx. You won't get hired, but if you are imaginative you can get the HR drone on the phone as you give them your details ("I don't like to send this information out via email, it's not secure") and ask them if they can view your posts OK.
Incidentally, if you use social media and you are employed, you should without question read the Evil HR Lady on how to avoid being fired.