While I have no actual knowledge why they made that decision, its not very hard to guess, and the possible answers are around their own staff familiarity with F

While I have no actual knowledge why they made that decision, its not very hard to guess, and the possible answers are around their own staff familiarity with FreeBSD, the pros of having the platform simplified by using single OS (Linux in their case, since for sure they've more servers with Linux already than they've acquired with WhatsApp) and all possible security and financial implications following these factors. However, it seems like at least for know they might be staying with FreeBSD a bit longer, what's a great PR/recognition win for the FreeBSD itself. I really doubt its for 'licencing issues' because (religious wars aside) the BSD license is way more attractive to businesses than GPL one. Said all that - I might be terribly wrong about everything I said, because its not an insider knowledge ;)