It'll be a small percentage of companies which actually bother to check. Even those who say they check probably don't. HR isn't some kind of overlord vetting the life out of applicants, they don't even check grades on your certificates half the time, and spend most of their time doing incredibly mundane **** like processing absence forms, leaving forms, application forms, dull data entry, sickness forms, payroll and administration. Hardly any companies have the kind of HR people are talking about here.
I do enjoy the 'this is what they like' stuff, though. Particularly the odd quasi-urban legends, like HR requires enough online presence to make it seem like you're not hiding, but not too much. As if there's a balancing act and they're all the same. Or my favourite (not so applicable here) that HR prefers a 2.1 to a first, because a 2.1 shows you 'have a life outside studying'. As a drunken photo does, I suppose.
The evidence available is largely common sense and anecdote. Everyone has a horror story of how their friend/cousin/dog/cat got shot in the middle of the office because they put three x's on the end of a Facebook status. Of the hundreds of people I've interviewed, I think I looked at one of them's Twitter because they (inexplicably - it was standard inane friendtalk) put it on their CV. I've known enough private sector employees to know that most of them don't have time to **** about on social media stalking their entry-level graduate applicants.
Of course, it's very much a 1984/panoptic scenario. You probably aren't being watched, but are you going to take the small risk that you might be?