You don't need people in a datacenter. You only need a skeleton crew for hands when things physically break, everything else should be done remotely
The frontend apps are all the same, whether they have 3 users or 3 billion.
You also end up leaving massive efficiencies on the floor by not hiring more specialists as you scale. This is a good read which explains it in detail https://danluu.com/in-house/
I would also suggest reading their published papers on in house systems to understand the challenges that come with operating at a large scale.
https://news.ycombinator.com/item?id=30940595
I’m sure FB could operate with a range of employees 40k-100k(?), but that number scaling with users is far from surprising.
Product and UI has its own complexities, but it seems clear each product is unique and would require different logic.
I’m still thinking it’s just enough to repair failures in a timely manner (and assuming there is spare capacity so you are not blocked if something breaks). Which means only a few hands on deck and the rest being triaged remotely.
Or not maybe we are still in the 90s when you need someone sitting at a desk in a server room to handle password resets :P