They're best suited for people who had a good experience with a BSD in the 90s and are sentimental about that. Otherwise, there's really no reason to go with them over a Linux system.
WhatsApp also used a BSD IIRC but I imagine they’ve transitioned to Meta’s standard stack by this point.
Yes, that happened. I was there. FreeBSD is great and we would have continued to use it, but as an aquisition, you can only push back on so much of the incumbent tech stack. Much of the team had experience at Yahoo and saw how hard it is for acquisitions to run in the same infrastructure if they're running a different OS, so we spent zero time asking to run FreeBSD at Facebook.
The hardware at Facebook was quite a bit different, so there was never an apples to apples comparison to say whether one OS (as tuned) was better than the other at the use case. They clearly both work, and I've got my opinions and other people have theirs, and that's fine.
[1]: https://en.wikipedia.org/wiki/Nintendo_Switch_system_softwar...
[2]: https://en.wikipedia.org/wiki/PlayStation_3_system_software
https://wiki.freebsd.org/Myths#FreeBSD_is_Just_macOS_Without...
https://developer.apple.com/library/archive/documentation/Da...
You have also had patches go from Apple straight into the FreeBSD tree with improvements they made on their side. So, yes, it certainly is not as easy a case as one would make for Sony’s forks, but the answer is certainly not a resounding “no”.