Legal experts in the US have suggested that researching a job applicant online is akin to interviewing them and as such could be violating employment and privacy laws. (Picked up via @jdthurber) I’m not sure this issue is being discussed particularly in the UK, either from the employer or the employee perspective. I think clarity on the legalities and liabilities here would be beneficial.
The website http://www.HRlaw.co.uk touches on the area of social media background checks in the UK, but it seems this is something of a grey area in the law. Any CIPD members or employment law specialists reading this, please do feel free to comment.