At the scale of Meta, "down" is a nuanced concept. You are very unlikely to get every piece of functionality seizing up at once. What you are likely to get is some services ceasing to function and other services doing error-handling.

For example, if the service that authenticates a user stops working but the service that shows the login form works, then you get a complex interaction. The resulting messaging - and thus user experience - depend entirely on how the login page service was coded to handle whatever failure the authentication service offered up. If that happens to be indistinguishable from a failure to authenticate due to incorrect credentials from the perspective of the login form service, well, here we are.

At Meta's scale, there's likely quite a few underlying services. Which means we could be getting something a dozen or more complex interactions away from wherever the failures are happening.