It's not just about adding redundancies. Redundancies don't protect you against bugs, and they're itself very complex, so they introduce more opportunity for errors. Even with best redundancy, you'll have incidents from time to time.
What they did was purposefully remove redundancy they had, in order to be able to track people more (in order to profit more) and possibly scale easier. Doing nothing would have been easier but yet they still did it.
Bear in mind WhatsApp used to maintain a userbase of 200 million with 50 total employees (not even just developers).
Lol, I'd believe you when they did purchased the service but the countless amount of times it went down since then obviously proves that they cannot even maintain the services when they are folded together on the same infrastructure.
It was a long time ago Facebook employed the best of the best. Seems like it's mostly average developers and infrastructure people there now just trying to hold up the house of cards they built.