$2 billion is not staggering amount. Given it has over 300 Million active users - thats just $6~$7 for user acquisition.
If you want to build twitter today - its definitely going to cost more. It has built a brand - and hundreds of millions of loyal fans.
Question is - is it worth the valuation thats put on the company. Given now they are cash flow positive - that they have lost $2bn in the past is immaterial.