Have you ever used ReactJS? That's being developed by Facebook in the majority.
Then there's a lot of graph theory being applied to your network connections to determine who are you likely to know. A lot of analysis of your browsing habits on FB (and off it, admittedly) to determine what ads you are likely to click on and what pages/communities you may be interested in.
Like it or not, Facebook is doing a LOT of research to keep itself relevant, both in engineering and in the minds of people.