The revenue comes from investors. A lot of those investors are saying the same thing you are: "Look at all this revenue, they must be doing well!" For those people it's a closed cycle: investors see revenue increase and invest because they see revenue increase, other investors see revenue increase and invest because they see revenue increase.

But this isn't a sustainable business, it's basically an unintentional pyramid scam. Businesses need to make a profit in order for people to receive a return on investment, and Twitter has never, not once, ever made a profit.