What's the revenue model?

Twitter CEO Jack Dorsey was asked what the revenue model was for Twitter and he replied: "I don't know. I'm making this up as I go,"
How do companies such as this raise 10's of millions? Is this 1999 all over again.

I started WrapMail in 2005 (actually started operations in 2006) and right our of the gate there was a very clear revenue model and pitch:
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You have a website and your employees send emails every day.
WrapMail makes every external email into a marketing tool without anything to install on any desktop – nothing to learn BUT when the email arrives the recipient is now exposed to your interactive letterhead!
Every external email can show your brand, your products/services and of course the images are clickable so you will drive more people to your website.
When your wrapped emails are clicked the system tells you who is clicking on what and when!
You’re going to send emails every day anyway, why not show people what you have to offer – just click on this email to experience for yourself. Every employee becomes part of the marketing effort with WrapMail.
Cost: $40 to sign up, thereafter $5 per sender per month
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We're approaching 1b corporate email accounts worldwide, pretty easy to multiply this with $5/month to see a colossal market. An added bonus is the fact that our model is residual!

I can understand that start ups fly by the seats of their pants in the beginning BUT there's got to be more to it than just building an audience, at least for me to invest.