I also wondered that before but then I reached and finally broke the Facebook's Algorithm.
It is NOT the person(s) who stalks to you frequently that shows on the side bar. It is the tags, comments, likes and interaction to friends are the factors. THIS IS THE ALGORITHM OF FACEBOOK. THERE IS NO BASIS FOR STALKERS. In my own facebook activities, I am always and everyday visiting a one friend's profile but I, myself doesn't show frequently to her sidebar. "Only once in a blue moon!!!", and that's because of the tags, comments, likes and interaction with her. Nothing to do with the stalking or visiting to her profile.
And as Facebook, themselves already said there is no way we can find out who is(are) seeing our profile. Which means the tagging, commenting and liking interaction and activities are only the factors why these friends keep showing on our side bar.
Source(s):
my experience, analysis and opinion