I've looked at Facebook profiles when judging potential employees. I've never ruled anyone out because of it, but one person got a lot closer to the job when I saw that that he spent his off hours talking about what he'd spend his work hours doing.
Basically, there are three possibilities when an employer looks at your Facebook: a) they like what they see (so they have better information, and you're more likely to get a job, b) they are neutral (nobody loses), or c) they dislike it (so by denying them access, you're artificially boosting your prospects -- cheating them out of the employee they want, and cheating that employee out of a job).
It's good to be suspicious of people trolling for personal data -- but I'm much more suspicious of efforts to hide information.