It's about revenue, not user count. Money changes everything: when customers pay, they expect a different level of service.

A company's revenue per employee is a common benchmark, and is usually around $600K-$1MM or so for successful companies at or near scale, up to $2.2MM for outliers like Apple(1). A company cannot grow to $100MM revenue and higher without having a large number of employees just to manage those revenue streams, partnerships, contracts, and support issues.

I should add that Instagram had ~13 employees, not two. That is a very low number, but mobile apps are easier to scale, support-wise, than web apps – users are just less likely to bug you, especially with a free app.

(1) Comparing Google, Microsoft, Apple and Amazon's Revenue per Employee: http://www.wolframalpha.com/input/?i=revenue+per+employee+ap...