I doubt it was snapshotting. It is common to force OS updates. For macOS there are tools where the employee will be prompted to update, and can dismiss for a while, but eventually they will be forced to restart. My guess is that the Facebook employee had ignored update prompts and was then forced to update. Every day does seem odd, I can't think of an update type that would require restarts/logoff every day. At least a couple years ago Facebook and Dropbox were managing their Macs in similar ways, with similar tools, I'd be surprised if they changed to something that required daily restarts.

Facebook has a Chef cookbook called cpe_nightly_reboot for their Macs. I can't think of why that would be required, but they must have a reason.

https://github.com/facebook/IT-CPE/tree/master/chef/cookbook...

I don't work at Facebook, but I'm fairly certain this is for things like build servers, not client machines.

They use Munki for software updates, which, just like you describe, gives the user a grace period where they can reboot at their leisure. Ignore it for long enough and it'll crank up the nagging, and eventually force a reboot.

Yes. Services that can run without a display are better run on a Linux server anyway.

I assumed it was for compilation of iOS apps, do those need a display?

No, you can compile iOS apps from the command line.

I don't think it's possible not to.