[...] we have begun work on systems to enable us to take action against
    terrorist accounts across all our platforms, including WhatsApp and Instagram.
    Given the limited data some of our apps collect as part of their service,
    the ability to share data across the whole family is indispensable to our
    efforts to keep all our platforms safe.

Looks like they are indeed linking WhatsApp and Facebook accounts internally. Weren't they fined for failing to disclose this pre-acquisition a couple weeks ago?