I don't use Facebook myself for a few years already, which is paradoxical as they were headhunting me recently to lead one of their ML teams and didn't mind that I quit them long time ago :D But maybe they were just harvesting my own "outlier" training dataset - who knows if I was filmed while onsite and now my gait is analyzed to smallest details including reconstructed skeleton and muscle attachments, my appearance is predicted for the next 30 years under various aging assumptions, maybe later used to fabricate realistically faked video etc. These things are now possible and not that difficult anymore.
Why not? You keep up to date, experiencing world-class hiring process and its requirements (IMO FB's hiring process is the best I've ever experienced, Google's feels one level down), you meet interesting people you might later work together on something cool either there or at another company, you visit interesting places, get some inspirations for your own businesses, interviews might be also a lot of fun (one interview I had at FB was an hour of one-joke-after-another while discussing/inventing architecture of some scalable system; imagine Cryptotinder which is a joke on its own) etc.
