Ads isn't just serving, but targeting. The better you target, the more your ads are worth. When you have $5B of advertising, a 0.01% improvement is breakeven for $500k of fully-loaded comp. So you should add as many engineers / data scientists / etc as can generate an 0.01% annual improvement. Or maybe you want to take 3x their annual salary: that's still an 0.03% improvement in ad relevance.

Separately, some commenters here are flatly delusional about the effort to ship a site, android and ios apps, internal mod tools, help docs, support, and legal docs in 34 supported languages. Not to mention obeying laws in all the countries that implies.

Or image and video hosting! With recoding of videos, resizing of images, and the management of what is surely petabytes of images and videos with very high reliability! That is not a 1, 2, or 3 person job to do well.