Facebook told me this is a group of relevant friends intended to be a useful prompt for the user.
In other words, these are the people the algorithm is subtly encouraging you to interact with. Or if someone else is visiting your Facebook page, it's a sampling of people that person might want to connect with based on your mutual friends or interests. This makes sense: It's in the best interest of Facebook's ad-driven business model for us to connect and engage with as many people as possible. [With 1.5 billion users, clearly the approach is working.]
Friend ordering algorithms take into account how much you "interact" with certain people as well as how often and how recently. So the random high school acquaintance that's been popping up in your highlighted friends could be the algorithm gently nudging you to keep in touch with someone you've been friends with a long time. On the flip side, it may show you people you recently friended to jumpstart the connection, even if you haven't interacted yet. The formula also gives extra weight to people who recently posted stories or photos, the company says.
And sometimes, yes, it's simply random, Facebook told me.
Facebook's algorithms are also constantly evolvingthis is the company's secret sauce and the ingredients are continually being tweaked to encourage engagement and promote new features. Until a couple years ago, Facebook used a friend algorithm called Edgerank. It looked at three factors to determine social proximity, affinity, weight, and decay: how much you interact, what kind of interactions, and how long ago.
The current formula is more complex. It uses machine learning and takes thousands of data points into account to determine people's social proximity. But assuming the basic tenets are the same, the equation likely weighs various types of interactions differently: Being tagged with someone in a photo or attending the same event is a better indicator that you're tight with them than liking a news story they shared or commenting on a wall post.
You can find some insight into the algorithm by scrolling through your activity log. Facebook tracks and records everything you do on the site, and if you browse through you can see all those little interactions you maybe forgot about, and draw a line between those and how the algorithm's ranking your friends. Those "random" folks showing up on your profile page or chat list may make a bit more sense.