They moved to Linux at some point after being bought by Facebook. Reason being, as best I recall, that Facebook thought it sensible to have homogenous hardware (they already had their own datacentres with many relatively low power Linux servers vs a relatively few high power FreeBSD servers that WhatsApp had grown up with). It caused a few challenges to migrate.
This is all gleaned from a talk from one of the WhatsApp folk and comes with the caveat that I may have misremembered.