This answer is prolly very late, but I feel like I have some input. So Sunny Kotharkar mentioned the following. 1) the frequency with which one checks the person's profile. 2) if you have already messaged the person. 3) if you have shared a photo tag with the person. read more
The friends that show up at the top of your sidebar are people you’ve interacted with frequently or recently on Facebook. You can't add friends to this part, though it updates dynamically as the people you interact with change. read more
Making people who use Facebook wonder if their crushes, crazy friends, boyfriends, girlfriends, ex's, etc. are "stalking" or "checking out" their facebook profile/pics when they show up on their Facebook sidebar. read more