It in fact scales exponentially.

Internal sources only, but topics like regionalization, localization, bespoke caching implementations, hardware-level optimizations, content moderation, policy adherence (e.g. GDPR, CCPA), long-term monetization (especially if supporting advertisers as a direct customer), 3P support, public APIs, documentation, SRE (a fledgling product doesn't need 5 9s), analytics (internal and for advertisers and 3P partners) and for the most part, security, are non-exhaustive examples of things you can mostly ignore when your product supports 10M users that are unavoidable when your product supports half of the US, a good chunk of the rest of the world, and other large businesses that consume you at large business scale.

You listed a bunch of requirements without an argument that fulfilling those requirements actually entails an exponential growth in employees.