I wonder, how did Quora persuaded their investors as they don't make revenue and even they don't have any revenue model as far as I know.
The question is same again for twitter ( I know they sell features tweets but this feature didn't exist in the early stage of twitter)
At the most basic level, the argument to investors is "we are building something that millions of people will use. Once they do, making money is the easy part."
Which frankly is a valid argument. If you build something that everyone wants, money will find you. On the other hand, if you build something that is not really interesting to most (it's called a niche market), then you'd better have convincing unit economics that show you can still make a profit with a limited audience.
What Alain said, but only because in both cases the companies were founded by proven entrepreneurs / technologists.
With twitter, Biz Stone and Evan Williams had built and sold Blogger to Google.
Quora's founders were the CTO of Facebook and the creator of their developer platform; by the time Benchmark decided to invest, they already had a tremendous amount of the tech and social media A-list participating.