It may not be exact but it makes sense to me. Go to the People you may know list on facebook. Most of the list will contain people with mutual friends. Some of the people on the list you will know, or not, but have zero mutual friends. These are the people that might be checking out your profile.

Here is why I think this is the case - I went to a conference in Florida a couple of months ago. Two people from the conference recently made a linkdin request. They both are near the top of my list of people I might know on facebook. There is no way facebook would know that I have any connection to these people unless it came from a search. They must have also searched for my Facebook profile.

Other people on my "people you may know list" without mutual friends are former colleagues, an old girlfriend, and many of my former students.

Could I be right?