I would assume its massively overprovisioned at this point to cover first few weeks if not months of projected growth.

But still there probably is all sorts of juicy details that would be neat to hear about

One of the "fun" things when I worked at a big company was thinking through how to make the very first launched version of something work at millions-of-users scale. There is no organic growth in traffic from zero when a non-negligible subset of billions of active users will try it out right away.

But I think on net it was still an easier problem than scaling a startup product from zero, because the tooling and processes to support huge usage is already so mature at these big companies.

Meta has 3.8 billion monthly active users across its products.

A few million new people is less than 0.1% of the total. It's entirely within expected variance.

I think they're looking at it from a an individual product perspective. % users of an entire company can be extremely different than % growth of a new product, within that company.

My naive assumption is that they have some neat simulated user/load system to help identify any scaling issues that might arise when launching a service, and the extreme % growth. That 3.8 billion was spread over 15 years.

It’s just running on the shared meta infrastructure which is built to handle that scale