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 ;)

How could licensing issues be a problem?

I think people prefer a monoculture, even though the security idea of having multiple software stacks is appealing, thoroughly testing issues on two OS stacks is a lot more work, especially at Facebook scale.

Presumably they are tied into Linux for a lot of their internal infrastructure.

Facebook is almost certainly further evolving Linux's existing TCP/IP stack, not developing a new one.

I would rather guess it is for consistency reasons. People are more familiar with Linux than FreeBSD.

Similarly one of the reasons Facebook relies on PHP is because a lot of developers are familiar with it.