It's pretty easy to get numbers that big when you're Twitter's size. Salary and overhead (taxes, rent, food) for 1000 employees is probably on the order of $300-400m/year, and
server costs for a high traffic site like Twitter is probably on the order of $100m/year.
Does Twitter have anything near 1000 employees? I wouldn't expect that.
I'd also expect the sever and trafic cost to scale from a "minimal" of around $100k a year in proportion with revenue. That is, I don't see how that's a good target for investiment money.
As a second thought, that makes some sense. Regional offices do localization, sales (with support), and legal adaptation mostly. Any company that wants to sell on several countries need such stuff.
More than a hundred employees for each office looks oversized, but not extremely so.
The only question yet open is if Twitter needed all this infrastructure to become a viable business, of if they could open smaller and grow into that size organically. But well, even if Twitter didn't need it, it's reasonable to imagine that some business would need it.
I guess my question of what a company could spend 1 billion of investment on is answered.