Not just gang affiliations, slander/libel/defamation are all things that the we may say to our friends at a coffee shop, but with FB, Twitter, Google+ and LinkedIn these comments can have far reaching business implications. It is no longer a matter of a few people, but the wrong message can become viral to thousands even millions.

The Techrepublic advice to not write anything down that you don't want to haunt you is very good.
Quite a few candidates lie on their CV, social media is a simple and good way to reference check.

While I wholeheartedly disagree with the invasion of privacy of requesting account details, there is no harm in asking to see the general pages on their FB. Asking candidates to be a FB friend before making the job offer, if they say no, then don't make the offer. If there is no trust, then no job, they can learn about you and you them equally and equitably.

Some things should remain private, but if candidates have a demonstrated behavior that is counter to your business interests, this is also a person you don't want to hire.