Why does that person you met once at a braai three months ago keep popping up in your Newsfeed?
And why is the weird guy you sat next to in high school biology on your top friends list?
Facebook keeps their algorithms super secret, so there's no way we'll ever know for sure (and if someone figures them out, Mark Z and co will undoubtedly change them toot sweet).
But, as the folks at Motherboard point out, there are a few clues that might give us some answers...
In terms of the nine friends that feature in the little square box on the left-hand side of your profile page (and the ones you see all over your feed as a result), Facebook says "this is a group of relevant friends intended to be a useful prompt for the user."
'The Friends unit on profiles are not populated using any private information, like whether they’ve viewed your profile'
In other words, as Motherboard's Meghan Neal points out, Facebook is subtly encouraging you to interact with these nine friends.
"Or if someone else is visiting your Facebook page," she explains, "it’s a sampling of people that person might want to connect with based on your mutual friends or interests."
The social network's friend order formulae give weight to how much, how often and how recently you've interacted with certain people.
So that ex that keeps popping up in your highlighted friends box could simply be Facebook's way of gently nudging you to keep in touch with someone you’ve been friends for ages -- and is not necessarily because you've been visiting their profile incessantly since you broke up.
“Friends that show up in the Friends unit on profiles are not populated using any private information, like whether they’ve viewed your profile," a spokesperson for the tech giant said.
"Instead, we use a variety of public signals, like whether they’ve recently liked or commented on one of your posts, to choose which friends to show you there."
Conversely, your featured friends may show you people you recently became Facebook friends, even if you haven’t interacted yet.
Interestingly, Facebook also gives extra weight to people who recently posted stories or photos.
But here's the kicker: "And sometimes, yes," Meghan says Facebook informed her, "it’s simply random."
Sources: motherboard.vice.com, metro.co.uk