When I did a deepdive on Facebook ~5 months ago ("Inside the Facebook Engineering Culture"), I confirmed with people inside the company the company having ~75,000 global employees, ~32,000 working in tech.
While I can't answer the question "what does the company need them all for", consider that the company's products are used by ~3.6B people every month, meaning they have about one full-time employee for every ~45,000 users.
Compare this with Google (one employee for every ~29,000 users) or Twitter (one employee for every ~52,000 users). The ballpark is similar, and shows the trend of it being harder to have fewer employees per thousand users (Twitter is much smaller than Meta, and Google twice the size in employee count while reached ~20% more monthly users).
The company generated ~$117B in revenue in 2021, which is ~$1.46M per employee. Profits per employee (net income divided by employee count) was ~$480K/employee.
There will, naturally be a cost of revenue, cost of customer support (even if not all of it in-house).
Also worth considering that Meta is composed like several companies: Facebook, Whatsapp, Instagram, Messenger, Reality Labs and several smaller/larger bets operating pretty independently.
I'd flip your question around: what if Meta had fewer employees, what would it mean? It would mean:
1. Increasing their profitability even further
2. Likely less support for current customers, and less of an ability to invest in forward-looking initiatives
I'm sure shareholders of Meta would be happy with #1. For #2, short-term shareholders would be happy to see all of this go. Long-term shareholders would likely realize there would be a cost to cutting down on customer support and R&D and I would not expect them to support this, given the very high profits Meta already has.
All in all, I find it remarkable that the company can operate with such revenue and profit numbers, and I suspect they are already deliberately hiring employees more conservatively, to keep up this profitability rate.
As a side comment, I have my thoughts on how non-applicable the linked article is for software engineers at Meta, as discussed in this comment [1] on this thread.
[1] https://news.ycombinator.com/item?id=33150495